Magnetoresistance device and method of fabricating the same

ABSTRACT

A magnetoresistance device is provided for improving thermal stability of a magnetoresistance element by preventing interdiffusion between a conductor (such as a via and an interconnection) for connecting the magnetoresistance element to another element and layers constituting the magnetoresistance element. A magnetoresistance device is composed of a magnetoresistance element, a non-magnetic conductor providing electrical connection between said magnetoresistance element to another element, and a diffusion barrier structure disposed between said conductor and said magnetoresistance element, the magnetoresistance element including a free ferromagnetic layer having reversible spontaneous magnetization, a fixed ferromagnetic layer having fixed spontaneous magnetization, and a tunnel dielectric layer disposed between said free and fixed ferroelectric layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is related to a magnetoresistance deviceand method of fabricating the same, particularly, to a magnetoresistancedevice including a magnetic tunnel junction (MTJ), which exhibits atunneling magnetoresistance effect (TMR), and method of fabricating thesame.

[0003] 2. Description of the Related Art

[0004] A resistance of a magnetic tunnel junction, which consists of twoferromagnetic layers and a tunnel barrier layer (or a tunnel dielectriclayer) disposed therebetween, changes depending on the relativedirection of the magnetizations of the magnetic tunnel junction. Suchphenomenon is known as a tunnel magnetoresistance effect. Measuring theresistance of the magnetic tunnel junction allows the detection of thedirection of the ferromagnetic layers. Making use of suchcharacteristics of the magnetic tunnel junction, magnetoresistancedevices including magnetic tunnel junctions are used for magnetic randomaccess memories (MRAMs), which store data in a non-volatile fashion, andmagnetic read heads for hard disc drives.

[0005] Magnetoresistance elements, which include a magnetic tunneljunction, are typically composed of a fixed magnetic layer, a freeferromagnetic layer, and a tunnel dielectric layer disposed between thefixed and free magnetic layers. The fixed magnetic layer has spontaneousmagnetization whose direction is fixed, while the free magnetic layerhas spontaneous magnetization whose direction is reversible.

[0006] In order to tightly fix spontaneous magnetization, fixedferromagnetic layers are often formed to be in contact withantiferromagnetic layers. Exchange interaction provided by anantiferromagnetic layer tightly fixes the spontaneous magnetization of afixed ferromagnetic layer. In general, antiferromagnetic layers areformed of manganese-including antiferromagnetic materials, such as IrMn,and PtMn.

[0007] Furthermore, a free ferromagnetic layer is often composed of hardand soft ferromagnetic layers, the hard ferromagnetic layer being madeof ferromagnetic materials having high spin polarization ratios, and thesoft magnetic layer being made of soft ferromagnetic materials. Suchstructure of a free ferromagnetic layer reduces the coercive force ofthe spontaneous magnetization of the free ferromagnetic layer with anincreased magnetoresistance ratio (MR ratio) of the magnetic tunneljunction. Hard ferromagnetic layers are generally made ofcobalt-including ferromagnetic materials, such as Co and CoFe, whilesoft ferromagnetic layers are generally made of nickel-includingferromagnetic materials, such as NiFe.

[0008] One of the issues of magnetoresistance elements is thermalstability. Subjecting magnetoresistance elements to a high temperaturecauses interdiffusion between layers incorporated therein. Thisinterdiffusion deteriorates the characteristics of the magnetoresistanceelements, especially the magnetoresistance ratio. Patent document 1presents a problem concerning interdiffusion between hard and softferromagnetic layers. The essential point is that nickel included in thesoft ferromagnetic layer is diffused into the hard ferromagnetic layer.Diffusion of nickel into a hard ferromagnetic layer deteriorates themagnetoresistance element. Japanese Open Laid Patent Application No.P2000-20922A discloses that an oxide or nitride layer for preventinginterdiffusion is disposed between hard and soft ferromagnetic layers.Japanese Open Laid Patent Application No. Jp-A 2002-158381 addresses aproblem that manganese from manganese-including antiferromagneticsdiffuses into fixed ferromagnetic layers. This documents discloses atechnique for avoiding diffusion of manganese into a fixed ferromagneticlayer by incorporating two ferromagnetic layers and a dielectric oramorphous layer disposed therebetween into the fixed magnetic layer.Japanese Open Laid Patent Application No. Jp-A 2001-237471 discloses atechnique for improving thermal stability of magnetoresistance elementsby inserting magnetic oxide layers into fixed and free ferromagneticlayers. Japanese Open Laid Patent Application No. Jp-A-Heisei 10-65232addresses a problem of interdiffusion between a free ferromagnetic layerand a buffer layer disposed under the free ferromagnetic layer. Thisdocument discloses a technique for reducing the thermally induceddiffusion into the buffer layer and thereby improving thermal stabilityby disposing an atomic diffusion barrier layer formed of oxides,nitrides, carbides, borides, or fluorides between the free ferromagneticlayer and the buffer layer.

[0009] Japanese Open Laid Patent Application No. Jp-A 2002-74627discloses a technique for increasing electron reflectivity and therebyimproving thermal stability by disposing a high conductivity layer andan electron reflection layer which is substantially crystalline, andmainly includes an element different from the main element used for thehigh conductivity layer. This document also discloses that the electronreflection layer includes a first layer close to the free magneticlayer, and a second layer far from the free magnetic layer, the firstlayer consisting of an oxide of an element that is more easily oxidizedthan that consisting of the second layer.

[0010] Another issue of magnetoresistance elements is reduction incoercive forces of free ferromagnetic layers. A free ferromagnetic layercomposed of a layered structure including hard and soft ferromagneticlayers may not have the coercive force reduced sufficiently in the casethat an increased MR ratio is required.

[0011] Reduction of the coercive force can be achieved by reducing theproduct of magnetization M_(s) and thickness t of the free ferromagneticlayer (which is referred to as “the product M_(s)·t”, hereinafter). Anexplanation of the reason is given in the following. For a uniaxial freeferromagnetic layer, the coercive field thereof depends on a totalanisotropy field of the free ferromagnetic layer. For themagnetoresistance element having the size of the sub-micron order, atotal anisotropy field of the free ferromagnetic layer mostly arisesfrom the shape-induced anisotropy field of the free ferromagnetic layer.Therefore, the coercive force of the ferromagnetic layer isapproximately equal to the shape-induced anisotropy field Ha. For thiscase, the anisotropy field Ha is represented by the following equation(1):

H _(a)=4πM _(s)(N _(x) −N _(y)),  (1)

[0012] where M_(s) is the saturated magnetization of the freeferromagnetic layer, and N_(x) and N_(y) are demagnetization factors ofthe magnetoresistance element in the directions along the long and shortsides, respectively. For a given thickness of the free ferromagneticlayer, N_(x)−N_(y) increases as an increase in the ratio of the longside to the short side (or the aspect ratio), and this results in anincrease in the shape-induced anisotropy field H_(a). Furthermore, thereduction in the size of the free ferromagnetic layer increases theanisotropy field H_(a), because of the increase in the demagnetizationfactors as the reduction in the size of the free ferromagnetic layer.For a fixed aspect ratio, the anisotropy field H_(a) is approximatelydescribed by the following equation (2):

H_(a)=4πM _(s) ·t/W,  (2)

[0013] where W is the length of the short side of the free ferromagneticlayer, and t is the thickness of the free ferromagnetic layer. Theequation (2) indicates that the coercive force of the free ferromagneticlayer can be reduced by reducing the product M_(s)·t. In general,reducing the thickness t of the free ferromagnetic layer achieves thereduction in product M_(s)·t, and thereby reduces the coercive force ofthe free ferromagnetic layer.

[0014] Additionally, Zhang et al. discloses a magnetic tunnel junctionincluding an FeO_(x) layer disposed between an Al₂O₃ layer and a CoFelayer, in Applied Physics Letters vol.89, No. 19, 7 May 2001, pp.2911-2913.

[0015] Furthermore, Matsuda et al. discloses that magnetoresistances ofmagnetic tunnel junctions are reduced by a geometrical structure of thejunctions in Applied Physics Letters, vol. 77, No. 19, 6 November 2000,pp. 3060-3062.

[0016] In addition, Moodera et al. discloses magnetoresistances ofmagnetic tunnel junctions are increased by a geometrical structure ofthe junctions in Applied Physics Letters, vol. 69, No. 5, 29 July 1996,pp. 708-710.

[0017] Also, Ohnuma et al. discloses a technique for forming a highlyresistive soft magnetic film with granular metal consisting of cobaltbase alloy, iron base alloy, and a non-magnetic oxide or nitride.

[0018] The inventor of the present invention has discovered thatinterdiffusion between a magnetoresistance element and a conductorelectrically connecting the magnetoresistance element to other elementscauses an undesirable influence on the characteristics of themagnetoresistance element. Operating a magnetoresistance elementrequires electrical connections of the magnetoresistance element withother elements (such as transistors). Therefore, a magnetoresistanceelement is connected to conductors such as via contacts andinterconnections that are electrically connected to other elements. Ingeneral, such conductors are formed of aluminum, copper, tungsten, ortitan nitride, as is the case of other semiconductor integratedcircuits. Tantalum, ruthenium, zirconium, or molybdenum may be used fora conductor providing electrical connections between magnetoresistanceelements and other elements. The interdiffusion between amagnetoresistance element and a conductor that electrically connects themagnetoresistance element to other elements causes the following threeinfluences.

[0019] Firstly, the diffusion of material included in the conductor intothe magnetic tunnel junction reduces the MR ratio thereof. For aconductor including aluminum, diffusion of aluminum into the magnetictunnel junction is especially serious, because aluminum is diffused byapplying relatively low temperature. In another aspect, the diffusion ofthe material of the conductor is essential because the diffusion ofmaterial included in the conductor, especially aluminum, into theantiferromagnetic layer and the soft ferromagnetic layer promotes thediffusion of manganese from the antiferromagnetic layer and nickel fromthe soft ferromagnetic layer into the tunnel barrier layer.

[0020] Secondly, the diffusion of material included in themagnetoresistance element into the conductor, which electricallyconnects the magnetoresistance element with other elements, increasesthe resistance of the conductor. The increase in the resistance of theconductor deteriorates the SN ratio for detecting the resistance of themagnetic tunnel junction. Especially, since manganese included in theantiferromagnetic layer, and nickel in the soft ferromagnetic layer arediffused by relatively low temperature, the increase in the resistancecaused by the diffusion of manganese and nickel is of significance.

[0021] Thirdly, thermal diffusion of material of the free ferromagneticlayer into the conductor that electrically connects the freeferromagnetic layer with other elements makes it difficult to achievereduction in the coercive force through reducing the thickness t of thefree ferromagnetic layer. This is because diffusion caused by a thermaltreatment causes a large change in the characteristics thereof, and thusreduces operation reliability of the magnetoresistance element when thefree ferromagnetic layer is decreased in the thickness. FIG. 26 is agraph illustrating influences caused by thermal treatments on 4πM_(s)·tof free ferromagnetic layers having reduced thicknesses t. Thecharacteristics of the free ferromagnetic layers are obtained under thecondition described in the following; the structure of the samples issub./Ta(10 nm)/AlO_(x)/Ni₈₁Fe₁₉/Ta(10 nm) The AlO_(x) films are formedthrough oxidizing aluminum films having a thickness of 1.5 nm. Thethicknesses of the Ni₈₁Fe₁₉ films are selected out of the values of 3.0nm, 2.6 nm, and 2.2 nm. The Ni₈₁Fe₁₉ films deposited through sputtering.The temperature of the thermal treatment ranges between 250° and 400°C., and the duration is 30 minutes. Magnetizations M_(s) are measuredwith a vibration magnetometer.

[0022] As illustrated in FIG. 26, thermal treatment on the Ni₈₁Fe₁₉films having thicknesses t less than 30 nm causes drastic changes in4πM_(s)·t thereof; furthermore 4πM_(s)·t is remarkably decreased as thedecrease in the thickness t and the increase of the temperature of thethermal treatment. These samples exhibit poor repeatability. Asthus-described, thermal treatment destabilizes 4πM_(s)·t of the freeferromagnetic layer when the thickness thereof is reduced down to 3 nm.Such instability prevents reduction of the thickness t of freeferromagnetic layers.

[0023] There is a need for providing a technique for reducinginterdiffusion between a magnetoresistance element and a conductorproviding electrical connections between the magnetoresistance elementand other elements.

SUMMARY OF THE INVENTION

[0024] An object of the present invention is to provide a technology forfurther improving thermal stability of a magnetoresistance element.

[0025] Another object of the present invention is to provide atechnology for further improving thermal stability of amagnetoresistance element through preventing interdiffusion betweenlayers incorporated within the magnetoresistance element and a conductor(such as a via contact and an interconnection) that electricallyconnects the magnetoresistance element with other elements.

[0026] Still another object of the present invention is to provide atechnology for preventing a phenomenon in which a resistivity of aconductor that provides electrical connections between amagnetoresistance element and other elements is increased by diffusionof material included in layers incorporated within the magnetoresistanceelement, especially nickel and manganese, into the conductor.

[0027] Still another object of the present invention is to provide atechnology for a phenomenon in which characteristics of amagnetoresistance element is deteriorated by diffusion of materialincluded in a conductor that electrically connects the magnetoresistanceelement with other elements into the magnetoresistance element.

[0028] Still another object of the present invention is to provide atechnology which prevents a phenomenon in which characteristics of amagnetoresistance element is deteriorated by diffusion of manganese ornickel, which are included in the magnetoresistance element, into thetunnel dielectric layer of the magnetoresistance element, whilemaintaining magnetic or electrical coupling within the fixed and freeferromagnetic layers of the magnetoresistance element.

[0029] Still another object of the present invention is to provide atechnology for achieving reduction in the thickness of a freeferromagnetic layer to thereby reduce the coercive force thereof.

[0030] Still another object of the present invention is to provide atechnology for reducing the thickness of the free ferromagnetic layer,and reducing the coercive force thereof through preventing diffusion ofmaterial included in a free ferromagnetic layer.

[0031] Still another object of the present invention is to provide atechnology for reducing a coercive force with superior rectangularity ofthe magnetoresistance curve of a free ferromagnetic layer and reducedvariances of the coercive force, and thereby achieving reduction in anaspect ratio and size of the magnetoresistance element.

[0032] In one aspect of the present invention, a magnetoresistancedevice in accordance with the present invention is composed of amagnetoresistance element, a non-magnetic conductor that electricallyconnects the magnetoresistance element with another element, and adiffusion barrier structure disposed between the conductor and themagnetoresistance element, wherein the magnetoresistance elementincludes a free ferromagnetic layer having reversible free spontaneousmagnetization, a fixed ferromagnetic layer having fixed spontaneousmagnetization, and a tunnel dielectric layer disposed between the freeand fixed ferromagnetic layers. The conductor which electricallyconnects the magnetoresistance element is typically composed of a viacontact and an interconnection layer.

[0033] The diffusion barrier structure is designed to have a functionfor preventing at least one material of the conductor from beingdiffused into the magnetoresistance element. Furthermore, the diffusionbarrier structure is designed to have a function for preventing at leastone material of the magnetoresistance element from being diffused intothe magnetoresistance element.

[0034] It is important to design the diffusion barrier structure forpreventing both diffusions from the conductor into the magnetoresistanceelement and from the magnetoresistance element into the conductor. Oneof the diffusions from the conductor into the magnetoresistance elementand from the magnetoresistance element into the conductor causes anotherdiffusion. Therefore, the fact that an oxidization layer prevents bothof these diffusions is quite preferable in terms of improvement of themagnetoresistance element.

[0035] Such structure is effective in the case that the conductorincludes at least one element selected from among the group consistingof Al, Cu, Ta, Ru, Zr, Ti, Mo, and W.

[0036] The diffusion barrier structure is preferably formed of materialselected among from the group consisting of oxides, nitrides, andoxynitrides. These materials are easy to be formed densely, and thusprovide an improved diffusion barrier. The formation of the diffusionbarrier structure with conductive nitrides desirably reduces theresistance thereof, and thereby improves SN ratio of themagnetoresistance element. The diffusion barrier structure preferablyconsists of oxide of element having a free energy of oxide formationless than those of elements included in layers connected on top andbottom surfaces of the diffusion barrier structure. The diffusionbarrier structure preferably consists of nitride of element having afree energy of nitride formation less than those of elements included inlayers connected on top and bottom surfaces of the diffusion barrierstructure. Correspondingly, the diffusion barrier structure preferablyconsists of oxynitride of element having free energies of oxide andnitride formations less than those of elements included in layersconnected on top and bottom surfaces of the diffusion barrier structure.In detail, the diffusion barrier structure is preferably formed ofmaterial selected from among the group consisting of AlO_(x), MgO_(x),SiO_(x), TiO_(x), CaO_(x), LiO_(x), HfO_(x), AlN, AlNO, SiN, SiNO, TiN,TiNO, BN, TaN, HfNO, and ZrN.

[0037] In the case that the diffusion barrier structure is formed ofoxide, it would be preferable if the tunnel dielectric layer and thediffusion barrier structure are formed of the same material. Suchstructure effectively reduces the cost required for depositing thetunnel dielectric layer and the diffusion barrier layer. In this case,the diffusion barrier layer is preferably thinner than the tunneldielectric layer. Additionally, in order to increase the SN ratio ofdetecting the direction of the spontaneous magnetization of the freeferromagnetic layer, the through-thickness resistance of the oxide layeris preferably smaller than that of the tunnel dielectric layer.

[0038] When the conductor includes first and second conductors, thefirst conductor being electrically connected to the fixed ferromagneticlayer without involving the tunnel dielectric layer, and the secondconductor being electrically connected to the free ferromagnetic layerwithout involving the tunnel dielectric layer, the diffusion barrierstructure preferably includes first and second diffusion barrier layers,the first diffusion barrier layer being disposed between the firstconductor and the fixed ferromagnetic layer, and the second diffusionbarrier layer being disposed between the second conductor and the freeferromagnetic layer. Such structure effectively prevents both of the twokinds of interdiffusions: one between the fixed ferromagnetic layer andthe first conductor, and the other between the free ferromagnetic layerand the second conductor. The first and second diffusion barrier layersare preferably formed of material selected from among the groupconsisting of oxides, nitrides, and oxynitrides.

[0039] The magnetoresistance device may include a manganese-includinglayer. A layer formed of antiferromagnetic layer such as PtMn, and IrMnis a typical manganese-including layer. In this case, the oxide layer ispreferably disposed between the manganese-including layer and theconductor. In another embodiment, the magnetoresistance device mayinclude a nickel-including layer. A layer formed of magnetically softferromagnetic material such as NiFe is a typical nickel-including layer.In this case, the oxide layer is preferably disposed between thenickel-including layer and the conductor.

[0040] When the conductor includes a first conductor electricallyconnected to the fixed ferromagnetic layer without involving the tunneldielectric layer, the oxide layer includes a first oxide layer formed ofoxide and disposed between the fixed ferromagnetic layer, and themagnetoresistance element includes a manganese-includingantiferromagnetic layer connected to the fixed ferromagnetic layer, thenthe antiferromagnetic layer is preferably positioned between the fixedferromagnetic layer and the first oxide layer. This structureeffectively prevents the diffusion of manganese from theantiferromagnetic layer to the first conductor.

[0041] It would be preferable if the fixed ferromagnetic layer includesa ferromagnetic layer directly contacted with the tunnel dielectriclayer, and a composite magnetic layer disposed between the ferromagneticand antiferromagnetic layers, the composite magnetic layer being formedof mixture of non-oxidized metal ferromagnetic material as main materialand oxide of non-magnetic element more reactive to oxygen than the metalferromagnetic material as sub material. The composite magnetic layerhaving this structure effectively prevents the diffusion of manganeseincluded in the antiferromagnetic layer into the tunnel dielectric layerwhile allowing exchange interaction from the antiferromagnetic layer tothe ferromagnetic layer. This results in that the composite magneticlayer and the ferromagnetic layer functions as the fixed ferromagneticlayer, and that deterioration of the magnetic tunnel junction, which ispotentially caused by diffusion of manganese, is reduced. Theferromagnetic layer and the metal ferromagnetic material included in thecomposite magnetic layer preferably are formed of metal ferromagneticalloy mainly consisting of cobalt. Cobalt has a high spin polarizationratio and exhibits high oxidization-resistance, while being hard to bediffused because of its thermally stability.

[0042] The free ferromagnetic layer preferably includes a ferromagneticlayer directly contacted with the tunnel dielectric layer and acomposite magnetic layer connected to the ferromagnetic layer, whereinthe composite magnetic layer is formed of mixture of non-oxidized metalferromagnetic material as main material, and oxide material as submaterial, the oxide material being oxide of non-magnetic element morereactive to oxygen than the metal ferromagnetic material. The compositemagnetic layer having this structure is relatively magnetically softbecause of its reduced crystalline magnetic anisotropy, and thus allowsthe free ferromagnetic layer to be magnetically soft without includingnickel.

[0043] In the case that the free ferromagnetic layer includes nickel andthe conductor includes a second conductor electrically connected to thefree ferromagnetic layer without involving the tunnel dielectric layer,the oxide layer preferably includes a second oxide layer disposedbetween the free ferromagnetic layer and the second conductor. Thisstructure effectively prevents the diffusion of material included in thesecond conductor into the magnetoresistance element while preventing thediffusion of nickel included in the free ferromagnetic layer into thesecond conductor.

[0044] In this case, the second oxide layer is preferably in directcontact with a nickel-including ferromagnetic layer incorporated withinthe free ferromagnetic layer. This structure avoids the change in thecomposition of the nickel-including ferromagnetic layer, and therebyimproves the characteristics of the magnetoresistance element.

[0045] It is also preferable that the free ferromagnetic layer includesa first nickel-free ferromagnetic layer, a composite magnetic layer, anda nickel-including second ferromagnetic layer, the first ferromagneticlayer being directly contacted with the tunnel dielectric layer, whereinthe composite magnetic layer is formed of mixture of non-oxidized metalferromagnetic material as main material, and oxide material as submaterial, the oxide material being oxide of non-magnetic element morereactive to oxygen than the metal ferromagnetic material, and the secondferromagnetic layer being connected to the composite magnetic layer andmagnetically softer that the composite magnetic layer and the firstferromagnetic layer. The composite magnetic layer prevents diffusion ofnickel from the second ferromagnetic layer into the tunnel dielectriclayer, while allowing the second ferromagnetic layer to effect exchangeinteraction on the first ferromagnetic layer. This achieves amagnetically soft magnetic resistance device with reduced nickeldiffusion into the tunnel dielectric layer. The metal ferromagneticmaterial included in the first ferromagnetic layer and the compositemagnetic layer preferably consists of metal ferromagnetic alloy mainlyconsisting of cobalt. Cobalt has a high spin polarization ratio andexhibits high oxidization-resistance, while being hard to be diffusedbecause of its thermally stability.

[0046] The free ferromagnetic layer preferably includes a firstferromagnetic layer directly connected to the tunnel dielectric layer, afirst composite magnetic layer connected to the first ferromagneticlayer, a second composite magnetic layer, and a non-magnetic layerdisposed between the first and second composite magnetic layer toprovide antiferromagnetically coupling therebetween, the first andsecond composite magnetic layer includes mixture of non-oxidized metalferromagnetic material and non-magnetic metal oxide. The non-magneticlayer keeps the spontaneous magnetization of the second compositemagnetic layer antiparallel to those of the first composite magneticlayer and the first ferromagnetic layer, and thereby makes the freeferromagnetic layer magnetically softer. This structure allows the freeferromagnetic layer to be magnetically soft without including nickel.

[0047] In order to make the free ferromagnetic layer magneticallysofter, it is preferable that the free ferromagnetic layer furtherincludes a second ferromagnetic layer connected to the second compositemagnetic layer, which includes nickel and is magnetically softer thanthe first and second composite magnetic layers and the firstferromagnetic layer. Nickel included in the second ferromagnetic layermakes the free ferromagnetic layer magnetically softer. It should benoted that the first and second composite magnetic layers avoid aproblem that nickel is potentially diffused into the tunnel dielectriclayer. The first ferromagnetic layer, the metal ferromagnetic materialincluded in the first and second composite magnetic layers arepreferably formed of metal ferromagnetic alloy mainly consisting ofcobalt. Cobalt has a high spin polarization ratio and exhibits highoxidization-resistance, while being hard to be diffused because of itsthermally stability.

[0048] For the free ferromagnetic layer including the first and secondcomposite magnetic layers and the non-magnetic layer, it is preferablethat the free ferromagnetic layer further includes a third ferromagneticlayer disposed between the first composite magnetic layer and thenon-magnetic layer, and a fourth ferromagnetic layer disposed betweenthe second composite magnetic layer and the non-magnetic layer, thethird and fourth ferromagnetic layers being formed of metalferromagnetic layers mainly consisting of cobalt.

[0049] When the magnetoresistance device further includes a magneticbias element providing the free ferromagnetic layer with a bias magneticfield, the magnetic bias element being composed of a magnetic biasferromagnetic layer, and a magnetic bias antiferromagnetic layerincluding manganese and connected to the magnetic biasing ferromagneticlayer, then the oxide layer preferably includes a first oxide layerdisposed between the magnetic bias element and the free ferromagneticlayer, and a second oxide layer disposed between the magnetic biaselement and the second conductor.

[0050] In the case that the conductor includes a second conductorelectrically connected to the free ferroelectric layer without involvingthe tunnel dielectric layer, the diffusion barrier structure preferablyincludes a second diffusion barrier layer between the free ferroelectriclayer and the second conductor.

[0051] It is preferable that the second diffusion barrier layer isdirectly contacted with the free ferroelectric layer and the freeferroelectric layer has a thickness less than 3 nm; especially, it ispreferable that the product of the saturated magnetization and thicknessof the free ferroelectric layer is less than 3 (T·nm).

[0052] Preferably, the free ferroelectric layer includes anickel-including ferroelectric film and the second diffusion layer isdirectly contacted with the nickel-including ferroelectric film.

[0053] The free ferroelectric layer preferably includes a ferroelectriclayer directly connected to the tunnel dielectric layer, and amagnetization control structure composed of non-magnetic element andferromagnetic material included in the ferroelectric layer.

[0054] The magnetization control structure is preferably non-magnetic.

[0055] The magnetization control structure preferably consists of oxideor nitride of the ferroelectric material included in the ferroelectriclayer.

[0056] The non-magnetic material preferably consists of one or moreelements selected from Ru, Pt, Hf, Pd, Al, W, Ti, Cr, Si, Zr, Cu, Zn,Nb, V, Cr, Mg, Ta, and Mo. It is also preferable that the non-magneticmaterial is segregated at grain boundaries of the ferromagneticmaterial.

[0057] The free ferromagnetic layer is preferably formed so that thestress-induced and shape-induced magnetic anisotropies exhibit the axesof easy magnetization in the same direction.

[0058] Specifically, it is preferable that the free ferromagnetic layerextends in a first direction and has a positive magnetostrictionconstant with a compressive stress exerted thereon in a second directionperpendicular to the first direction. Alternatively, a tensile stress ispreferably exerted on the free ferromagnetic layer in the firstdirection.

[0059] In the case that the free ferromagnetic layer has a negativemagnetostriction constant, a compressive stress is preferably exerted onthe free ferromagnetic layer in the first direction; instead, a tensilestress is preferably exerted in the second direction perpendicular tothe first direction.

[0060] Control of the stress may be achieved by a relative direction ofthe major axis of the free ferroelectric layer and a lowerinterconnection. Specifically, in the case that the free ferromagneticlayer has a positive magnetostriction constant and the freeferroelectric layer extend in a first direction, the lowerinterconnection is formed to extend in the first direction.

[0061] In the case that the free ferromagnetic layer has a negativemagnetostriction constant and the free ferroelectric layer and tunneldielectric layer contact with each other on a contact interfaceextending in the first direction, which is perpendicular to the seconddirection, then the lower interconnection is formed to extend in thesecond direction.

[0062] The free ferromagnetic layer preferably has a stress-inducedmagnetic anisotropy larger than the shape-induced magnetic anisotropythereof. Such characteristics of the free ferromagnetic layer isespecially preferable for the case that the free ferromagnetic layer hasmajor and minor axes, the minor axis being perpendicular to the majoraxis, and an aspect ratio, which is defined as the ratio of the majoraxis to the minor axis, is equal to or more than 1.0 and equal to orless than 2.0.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063]FIG. 1 is a cross sectional view illustrating one embodiment ofthe magnetoresistance device in accordance with the present invention;

[0064]FIG. 2 is a cross sectional view illustrating a first modificationof the magnetoresistance device in this embodiment, including a thickbottom contact layer 12′;

[0065]FIG. 3 is a cross sectional view illustrating a secondmodification of the magnetoresistance device in this embodiment,including a thick top contact layer 15′;

[0066]FIG. 4 is a cress sectional view illustrating a third modificationof the magnetoresistance device in this embodiment, in which anantiferromagnetic layer 7′, a buffer layer 6′, and a seed layer 5′ areincorporated within an interconnection;

[0067]FIG. 5 is a cross sectional view illustrating a fourthmodification of the magnetoresistance device in this embodiment, inwhich a fixed ferroelectric layer 8 includes a composite magnetic layer8 a;

[0068]FIGS. 6A and 6B are cross sectional view illustrating thestructure of the composite magnetic layer 8 a;

[0069]FIG. 7 is a graph illustrating a resistivity of a thin film formedthrough sputtering an alloy target consisting of ferromagnetic CoFe, andnon-magnetic Ta with sputtering gas including oxygen gas;

[0070]FIG. 8 is a graph illustrating a saturated magnetization of thethin film;

[0071]FIG. 9 is a CO_(2p) spectrum obtained through an XPS analysis ofthe thin film;

[0072]FIG. 10 is a cross sectional view of a fifth modification of themagnetoresistance device in this embodiment, in which a freeferromagnetic layer 10 includes a composite magnetic layer 10 b;

[0073]FIG. 11 is a cross sectional view of a sixth modification of themagnetoresistance device in this embodiment, including a compositemagnetic layer 10 b and a soft ferromagnetic layer 10 b;

[0074]FIG. 12 is a cross sectional view illustrating seventhmodification of the magnetoresistance device in this embodiment, inwhich a free ferromagnetic layer 10 includes a metal ferromagnetic layer10 d, a composite magnetic layer 10 e, a non-magnetic layer 10 f, acomposite magnetic layer 10 g and a soft ferromagnetic layer 10 h;

[0075]FIG. 13 is a cross sectional view illustrating the seventhmodification of the magnetoresistance device in this embodiment, inwhich the free ferromagnetic layer 10 includes a metal ferromagneticlayer 10 i;

[0076]FIG. 14 is a cross sectional view illustrating a eighthmodification of the magnetoresistance device in this embodiment,including a magnetic bias layer 20;

[0077]FIG. 15 is a cross sectional view illustrating a ninthmodification of the magnetoresistance device in this embodiment,including a composite magnetic layer 8 a, a composite magnetic layer 10b, a soft ferromagnetic layer 10 c, a magnetic bias layer 20, and anoxide layer 21;

[0078]FIG. 16 is a cross sectional view illustrating a tenthmodification of the magnetoresistance device in this embodiment, inwhich a via contact 11 and tunnel ferromagnetic layer 9 do not overlapin a direction perpendicular to a major surface of a substrate 1;

[0079]FIG. 17 is a cross sectional view illustrating an eleventhmodification of the magnetoresistance device in this embodiment, inwhich a via contact 11 and tunnel ferromagnetic layer 9 do not overlapin a direction perpendicular to a major surface of a substrate 1;

[0080]FIG. 18 is a cross sectional view illustrating a twelfthmodification of the magnetoresistance device in this embodiment,including a write interconnection 2′ in addition to a bottominterconnection 2, which is used for read operation;

[0081]FIG. 19 is a graph illustrating dependencies of MR ratios ofmagnetic tunnel junctions on thermal treatment temperature obtained fromComparative Example 1 and Example 1;

[0082]FIG. 20 is a graph illustrating changes in sheet resistances ofAlCu layers depending on thermal treatment temperature obtained fromComparative Example 2 and Example 2;

[0083]FIG. 21 is a table illustrating changes in MR ratios of magnetictunnel junctions depending on thermal treatment temperature obtainedfrom Comparative Example 2, and Examples 2 and 3;

[0084]FIG. 22 is a graph illustrating changes in MR ratios of magnetictunnel junctions depending on thermal treatment temperature obtainedfrom Comparative Example 3 and Example 4;

[0085]FIG. 23 is a graph illustrating magnetization curves obtained fromComparative Example 3;

[0086]FIG. 24 is a graph illustrating magnetization curve obtained formExample 4;

[0087]FIG. 25 is a table illustrating changes in saturatedmagnetizations of free ferromagnetic layers obtained from ComparativeExamples 4 and 5, and Examples 5 and 6;

[0088]FIG. 26 is a graph illustrating influences on 4πM_(s)·t of a freeferromagnetic layer having a thin film thickness t, caused by thermaltreatment;

[0089]FIG. 27 is a cross sectional view illustrating a structure of afree ferromagnetic layer for reducing the product 4πM_(s)·t;

[0090]FIG. 28 is a cross sectional view illustrating another structureof a free ferromagnetic layer for reducing the product FIG. 29 is across sectional view illustrating still another structure of a freeferromagnetic layer for reducing the product 4πM_(s)·t;

[0091]FIG. 30 is a cross sectional view illustrating still anotherstructure of a free ferromagnetic layer for reducing the product4πM_(s)·t;

[0092]FIG. 31 is a cross sectional view illustrating an MRAM structurefor allowing shape-induced and stress-induced magnetic anisotropies toexhibit axes of easy magnetization in the same direction;

[0093]FIG. 32 is a plan view illustrating the MRAM structure forallowing shape-induced and stress-induced magnetic anisotropies toexhibit axes of easy magnetization in the same direction;

[0094]FIG. 33 is a plan view illustrating a relation betweencrystalline, shape-induced, and stress induced anisotropies;

[0095]FIG. 34 is a plan view illustrating another MRAM structure forallowing shape-induced and stress-induced magnetic anisotropies toexhibit axes of easy magnetization in the same direction;

[0096]FIG. 35 is a plan view illustrating a preferable relation betweenshape-included and stress-induced anisotropies;

[0097]FIG. 36 is a graph illustrating an influence on 4πM_(s)·t causedby thermal treatment obtained from Example 7;

[0098]FIG. 37 is a graph illustrating influences on 4πM_(s)·t caused bythermal treatment obtained from Comparative Example 7 and Examples 8, 9,and 10;

[0099]FIG. 38 illustrates magnetization curves of free ferromagneticlayers of Comparative Example 7 and Example 11;

[0100]FIG. 39A illustrates a magnetization curve of a free ferromagneticlayer of Comparative Example 8;

[0101]FIG. 39B illustrates a magnetization curve of a free ferromagneticlayer of Comparative Example 9;

[0102]FIG. 39C illustrates a magnetization curve of a free ferromagneticlayer of Example 12; and

[0103]FIG. 40 is a graph illustrating relations between aspect ratiosand yields obtained from Comparative Example 9 and Example 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0104] As shown in FIG. 1, one embodiment of the present inventionaddresses a cross-point cell type MRAM. In a first embodiment, bottomand top interconnections 2 and 3 are provided on an upper side of asubstrate 1. The bottom and top interconnections 2 and 3 are formed ofAl₉₀Cu₁₀.

[0105] A magnetoresistance element 4, which functions as a memory cellof an MRAM, is disposed between the bottom and top interconnections 2and 3. The magnetoresistance element 4 includes a seed layer 5, a bufferlayer 6, an antiferromagnetic layer 7, a fixed ferromagnetic layer 8, atunnel dielectric layer 9, and a free ferromagnetic layer 10. The fixedferromagnetic layer 8, the tunnel dielectric layer 9, and the freeferromagnetic layer 10 within the magnetoresistance element 4 form amagnetic tunnel junction.

[0106] The fixed ferromagnetic layer 8 is formed of metal ferromagneticalloy having a high spin polarization ratio, typically CoFe. CoFe alloyis relatively magnetically hard ferromagnetic material; the term “hard”means to have a large coercive force. As described later, spontaneousmagnetization of the fixed ferromagnetic layer 8 is fixed by exchangeinteraction from the antiferromagnetic layer 7.

[0107] The free ferromagnetic layer 10 is formed of relativelymagnetically soft ferromagnetic material; the term “soft” means to havea small coercive force. The free ferromagnetic layer 10 is formed toallow the direction of spontaneous magnetization thereof to bereversible in the directions parallel and antiparallel to that of thefixed ferromagnetic layer 8. The magnetoresistance element 4, in whichthe spontaneous magnetization of the free ferromagnetic layer 10 isreversible, stores therein data of one bit as the direction of thespontaneous magnetization of the free ferromagnetic layer 10.

[0108] The free ferromagnetic layer 10, which has reversible spontaneousmagnetization, is formed of nickel-including ferromagnetic material,typically, NiFe. In general, nickel-including ferromagnetic material isrelatively magnetically soft, and thus is preferable for providing thefree ferromagnetic layer 10 having reversible spontaneous magnetization.The free ferromagnetic layer 10 may be composed of a CoFe layer formedon the tunnel dielectric layer 9, and a NiFe layer formed on the CoFelayer. The CoFe layer, having a high spin polarization ratio, improvesan MR ratio of the magnetic tunnel junction, while the NiFe layer makesthe CoFe layer magnetically soft to reduce the coercive force. Thelayered structure composed of the CoFe layer and NiFe layer achieves amagnetic tunnel junction that has easily reversible spontaneousmagnetization and has a high MR ratio.

[0109] The tunnel dielectric layer 9 is formed of non-magneticdielectric having a thin thickness to allow a tunnel current to flowtherethrough. The tunnel dielectric layer 9 is typically formed ofAlO_(x), AlN_(x), or MgO_(x); the thickness thereof is adjusted on thebasis of the resistance required for the magnetoresistance element 4,typically, 1.2 to 2 nm. Because of the tunnel magnetoresistance effect(TMR effect), the through-thickness resistance of the tunnel dielectriclayer 9 depends on whether the spontaneous magnetizations of the fixedand free ferromagnetic layers 8 and 10 are parallel or antiparallel.Data stored in the magnetoresistance element 4 can be distinguished onthe basis of the through-thickness resistance of the tunnel dielectriclayer 9.

[0110] The seed layer 5 and buffer layer 6 controls the orientation ofthe antiferromagnetic layer 7, which is disposed thereon, to stabilizethe antiferromagnetic phase of the antiferromagnetic layer 7. The seedlayer 5 is typically formed of Ta or Cr, while the buffer layer 6,disposed on the seed layer 5, is typically formed of NiFe or CoFe.

[0111] The antiferromagnetic layer 7 is formed of manganese-includingantiferromagnetic material, typically PtMn, or IrMn. Theantiferromagnetic layer 7 fixes the spontaneous magnetization of thefixed ferromagnetic layer 8 with exchange interaction thereon.

[0112] A via contact 11, a bottom contact layer 12, and an oxide layer13 are disposed between the magnetoresistance element 4 and the bottominterconnection 2. The fixed ferromagnetic layer 13 within themagnetoresistance element 4 is electrically connected to the bottominterconnection 2 through the via contact 11, the bottom contact layer12, and the oxide layer 13. The via contact 11 is connected to thebottom interconnection 2 to extend in a direction perpendicular to themajor surface of the substrate 1. The via contact 11 is typically formedof tungsten, copper, or molybdenum.

[0113] The bottom contact layer 12 is disposed on the via contact 11.The bottom contact layer 12 functions as a superior adhesive layer ontothe via contact 11, and also improves the film quality of the oxidelayer 13 disposed thereon. Additionally, the bottom contact layer 12provides a superior electrical connection between the via contact 11 andthe oxide layer 13. The bottom contact layer 12 is typically formed ofTiN, Ta, Ru, W, Zr, or Mo.

[0114] The oxide layer 13 effectively prevents interdiffusion betweenthe magnetoresistance element 4 and the underlying structure: the bottominterconnection 2, the via contact 11, and the bottom contact layer 12.That is, the oxide layer 13 effectively prevents the magnetoresistanceelement 4 from being diffused with aluminum or copper from the bottominterconnection 2, tungsten, copper or molybdenum from the via contact11, TiN, Ta, Ru, W, Zr, or Mo from the bottom contact layer 12.Furthermore, the oxide layer 13 effectively prevents the bottominterconnection 2 from being diffused with Ni from the buffer layer 6,and Mn from the antiferromagnetic layer 7. The oxide layer 13 formed ofoxide, which is easily fine-structured, effectively prevents theinterdiffusion.

[0115] It is essential that the oxide layer 13 has a function forpreventing both diffusions from the bottom interconnection 2, the viacontact 11, and the bottom contact layer 12 into the magnetoresistanceelement 4 and from the magnetoresistance element 4 into the bottominterconnection 2, the via contact 11, and the bottom contact layer 12;one of the diffusions causes the other diffusion. Therefore, the factthat the oxide layer 13 prevents both of these diffusions is preferablein terms of improvement of the characteristics of the magnetoresistanceelement 4.

[0116] The oxide layer 13 is preferably formed of oxide of Al, Mg, Si,Hf, Li, Ca, or Ti. Forming the oxide layer 13 with oxide of any of theseelements allows the oxide layer 13 to be highly fine-structured andthermally stable because of the strong reaction with oxygen; thiseffectively suppresses the interdiffusion.

[0117] The use of a film of oxide is especially preferable foreffectively suppressing the diffusion of manganese. Because manganese ishighly reactive to oxygen, manganese diffused into the oxide layer 13,formed of oxide, reacts with oxygen to be stabilized and fixed withinthe oxide layer 13. Stabilizing manganese in the oxygen layer 13effectively prevents the diffusion of manganese into the bottominterconnection 2.

[0118] The oxide layer 13 is preferably formed of oxide of material morereactive to oxygen than material included in layers in contact with thebottom and top surfaces of the oxide layer 13 (that is, the bottomcontact layer 12 and the seed layer 5), the bottom surface designatingthe surface on the side of the substrate 1. The use of oxide of materialnot satisfying this requirement causes diffusion of oxygen into thelayers in contact with the bottom and top surfaces of the oxide layer 13and thereby destabilizes the oxide layer 13; this undesirablyinvalidates the anti-diffusion effect of the oxide layer 13. In the casethat tantalum is used for the bottom contact layer 12, and tantalum orchromium is used for the seed layer 5, the oxide layer 13 is preferablyformed of oxide of aluminum, magnesium, silicon, hafnium, lithium,calcium, or titanium; these elements have larger free energies of oxideformation.

[0119] In order to improve an SN ratio for detecting the resistance ofthe tunnel dielectric layer 9, the resistance of the oxide layer 13 inthe thickness direction is preferably minimized. The resistance of theoxide layer 13 in the thickness direction reduces the SN ratio ofdetecting the resistance of the tunnel dielectric layer 9, because theoxide layer 13 is connected in series to the magnetic tunnel junction.Accordingly, the resistance of the oxide layer 13 in the thicknessdirection is preferably small, more specifically, preferably smallerthan that of the tunnel dielectric layer 9.

[0120] It is preferable that the oxide layer 13, which functions as adiffusion barrier layer, is thin even when the resistivity thereof issmall (that is, even if the oxide layer 13 is allowed to have a largethickness). Thickly depositing the oxide layer 13 enlarges the latticedistortion of the oxide layer 13, and thereby undesirably exerts stresson the free ferromagnetic layer 10. The stress exerted on the freeferromagnetic layer 10 changes the magnetic anisotropy of the freeferromagnetic layer 10, and thereby causes difficulties in preferablycontrolling the characteristics of the free ferromagnetic layer 10.Specifically, the oxide layer 13 preferably has a thickness less than 5nm.

[0121] Additionally, reducing the thickness of oxide layer 13 below 1 nmin preferable because it substantially eliminates the resistance of theoxide layer 13 in the thickness direction. Reducing the thickness ofoxide layer 13 below 1 nm extremely reduces the resistance of the oxidelayer 13 through the tunneling phenomenon.

[0122] It is preferable to form the tunnel dielectric layer 9 and theoxide layer 13 with the same material because this allows the formationof the tunnel dielectric layer 9 and the oxide layer 13 using the sameapparatus and material, and thus reduces the fabrication cost of theMRAM. For the formation of the tunnel dielectric layer 9 and the oxidelayer 13 through sputtering, for example, the formation of the tunneldielectric layer 9 and the oxide layer 13 with the same material allowsthe deposition of the tunnel dielectric layer 9 and the oxide layer 13with the same sputtering target.

[0123] In the case that the tunnel dielectric layer 9 and the oxidelayer 13 are formed with the same material, the thickness of the oxidelayer 13 is preferably thinner than that of the tunnel dielectric layer9. This reduces the resistance of the oxide layer 13 in the thicknessdirection below that of the tunnel dielectric layer 9, and therebyimproves the SN ratio for detecting the resistance of the tunneldielectric layer 9.

[0124] An oxide layer 14 and a top contact layer 15 are disposed betweenthe top interconnection 3 and the magnetoresistance element 4. The oxidelayer 14 is formed on the free ferromagnetic layer 10 of themagnetoresistance element 4. The top contact layer 14 is formed on theoxide layer 14 to be in contact with the top interconnection 3. The freeferromagnetic layer 10 of the magnetoresistance element 4 iselectrically connected to the oxide layer 14 and the top contact layer15.

[0125] The top contact layer 15 protects the layers underlying below itfrom damages caused by the element fabrication process, and alsoprovides an improved electrical connection between the oxide layer 14and the top interconnection 3. The top contact layer 15 is typicallyformed of TiN, Ta, Ru, W, Zr, or Mo.

[0126] The oxide layer 14 effectively prevents the interdiffusionbetween the top interconnection 3 and the magnetoresistance element 4.In other words, the oxide layer 14 prevents the magnetoresistanceelement 4 from being diffused with aluminum and copper included in thetop interconnection 3, and also prevents the top interconnection 3 frombeing diffused with nickel included in the free ferromagnetic layer 10.Reducing the diffusion of nickel into the top interconnection 3 avoidsan increase in the resistance of the top interconnection 3. The oxidelayer 14, formed of oxide, is easily fine-structured, and thuseffectively prevents the interdiffusion. As is the case of the oxidelayer 13, the oxide layer 14, formed of oxide, is desirably reduces thediffusion of nickel, effectively.

[0127] It is of importance that the oxide layer 14, which is directlyconnected to the free ferromagnetic layer 10, prevents the topinterconnection 3 from being diffused with nickel from the freeferromagnetic layer 10, because this allows the free ferromagnetic layer10 to have a reduced thickness t, and thereby to exhibit a reducedM_(s)·t. Especially in the case that the free ferromagnetic layer 10 hasa reduced thickness t, the diffusion of nickel from the freeferromagnetic layer 10 causes a change in the composition of the freeferromagnetic layer 10, and thus destabilizes the characteristics of thefree ferromagnetic layer 10. Disposing the oxide layer 14 avoids nickelbeing diffused from the free ferromagnetic layer 10, and thereby enablesthe composition of the free ferromagnetic layer to be regulated to adesired value. This is especially effective for the case that the freeferromagnetic layer 10 has a reduced thickness t.

[0128] As is the case of the oxide layer 13, it is of importance thatthe oxide layer 14 has a function of preventing both diffusions from thetop interconnection 3 into the free ferromagnetic layer 10 and from thefree ferromagnetic layer 10 into the top interconnection 3. It isextremely preferable that the oxide layer 13 prevents both of thesediffusions in terms of improvement of the characteristics of themagnetoresistance element 4.

[0129] Characteristics required for the oxide layer 14 are identical tothose for the oxide layer 13; the preferred materials and structures forthe oxide layer 13 are also preferable for the oxide layer 14. Firstly,in order to be fine-structured, to exhibit thermal stability for hightemperature, and to thereby provide an improvedinterdiffusion-resistance, the oxide layer 14 is preferably formed ofoxide of aluminum, magnesium, silicon, hafnium, lithium, calcium, ortitanium. Furthermore, the oxide layer 14 preferably has a resistance inthe thickness direction smaller than that of the tunnel dielectric layer9. Additionally, reducing the thickness of the oxide layer 14,especially below 5 nm, is preferable for the suppression of theinfluences of the stress on the free ferromagnetic layer 10. Finally,the tunnel dielectric layer 9 and the oxide layer 14 are preferablyformed with the same material, because this allows the deposition of thetunnel dielectric layer 9 and the oxide layer 14 with the same apparatusand material, and thereby reduces the fabrication cost of the MRAM.

[0130] In the case that the free ferromagnetic layer 10 is formed with anickel-including ferromagnetic layer (typically NiFe layer), or composedof a layered structure of a nickel-including ferromagnetic layer andanother ferromagnetic layer (typically a layered structure of CoFe andNiFe layers), the oxide layer 14 is preferably formed to be directlyconnected with the nickel-including ferromagnetic layer. Changes in thecomposition of nickel-including ferromagnetic layers from the optimizedcomposition cause severe deterioration of the characteristics thereof.Saturated magnetization, for example, remarkably depends on theconcentration of nickel. Disposing the oxide layer 14 directly on thenickel-including ferromagnetic layer eliminates the diffusion route ofnickel upward from the ferromagnetic layer, and thereby effectivelyprevents the change in the composition of the nickel-includingferromagnetic layer. The inventors' experiments have proved that thenickel-including ferromagnetic layer experiences the diffusion into thelayers connected with the top surface thereof (that is, the top contactlayer 15 and the top interconnection 3) more remarkably than thediffusion into the layers connected with the bottom surface thereof(that is, the tunnel dielectric layer 9 or the aforementioned otherferromagnetic layer); therefore, disposing the oxide layer 14 directlyon the nickel-including ferromagnetic layer is especially effective forreducing diffusion.

[0131] As thus described, this embodiment provides the oxide layer 13between the bottom interconnection 2 and magnetoresistance element 4,and thereby effectively prevents the interdiffusion between the bottominterconnection 2 and magnetoresistance element 4. Additionally, thisembodiment provides the oxide layer 14 between the top interconnection 3and magnetoresistance element 4, and thereby effectively prevents theinterdiffusion between the top interconnection 3 and magnetoresistanceelement 4. The use of the oxide layers 13 and 14 are also effective forthe case that the bottom and top interconnections 2 and 3 are formedwith copper.

[0132] Furthermore, this embodiment provides the oxide layer 14 on thefree ferromagnetic layer 10, and thereby effectively prevents thediffusion of material of the free ferromagnetic layer 10, especially,nickel. This enables the reduction of the product M_(s)·t of themagnetization M_(s) and thickness t of the free ferromagnetic layer 10down to a small value even if the thickness t of the free ferromagneticlayer 10 is reduced. As described above, the reduction in the productM_(s)·t is effective for reducing and stabilizing the coercive force ofthe free ferromagnetic layer 10. The structure described in thisembodiment is especially effective for reducing the thickness t of thefree ferromagnetic layer 10 below 3 nm, and the product M_(s)·t below 3(T·nm).

[0133] In this embodiment, the oxide layers 13 and 14 may be replacedwith nitride layers. Nitride layers or oxynitride layers are easilyfine-structured, and thus effectively prevent the interdiffusion. As isthe case of the oxide layers 13 and 14, the nitride layers preferablyhave a thickness equal to or less than 5 nm. Increase in the thicknessof the nitride layers is not desirable because stress-induced magneticanisotropy may cause deterioration of the magnetic characteristics.

[0134] Forming the aforementioned nitride layers with conductive nitridedesirably reduces the resistance of the nitride layers in the thicknessdirection, and thereby improves the SN ratio for detecting theresistance of the tunnel dielectric layer 9. The resistance of thenitride layers in the thickness direction is preferably reduced,specifically, reduced below the resistance of the tunnel dielectriclayer 9 in the thickness direction.

[0135] The nitride layers are preferably formed with nitride of materialhaving a free energy of nitride formation smaller than that of materialincluded in the layers connected on the bottom and top surfaces thereof;the bottom surface designates the surface on the side of the substrate1. The use of nitride material not satisfying this requirementundesirably destabilizes the nitride layers through the diffusion ofnitrogen into the layers connected on the bottom and top surfaces of thenitride layers, and thus degrades the diffusion-resistance of thenitride layers. The preferable nitride used for the nitride layersincludes AlN, SiN, TiN, BN, TaN, and ZrN.

[0136] Alternatively, the oxide layers 13 and 14 may be replaced withoxynitrides layers formed with oxynitride in this embodiment. Oxynitridelayers are easily fine-structured, and thus effectively prevent theinterdiffusion. As is the case of the nitride layers, the oxynitridelayers preferably have a thickness equal to or less than 5 nm. Increasein the thickness of the nitride layers is not desirable becausestress-induced magnetic anisotropy may cause deterioration of themagnetic characteristics.

[0137] The oxynitride layers are preferably formed with oxynitride ofmaterial having free energies of oxide and nitride formations smallerthan those of material included in the layers connected on the bottomand top surfaces thereof; the bottom surface designates the surface onthe side of the substrate 1. The use of oxynitride material notsatisfying this requirement undesirably destabilizes the oxynitridelayers through the diffusion of oxygen and/or nitrogen into the layersconnected on the bottom and top surfaces of the oxynitride layers, andthus degrades the diffusion-resistance of the oxynitride layers. Thepreferable nitride used for the oxynitride layers includes AlN, SiN,TiN, BN, TaN, and ZrN.

[0138] As described above, disposing the oxide layer 14 (or the nitrideor oxynitride layer) on the free ferromagnetic layer 10 achieves thereduction of the thickness t of the free ferromagnetic layer 10, andthereby achieves the reduction of the product M_(s)·t of themagnetization M_(s) and thickness t of the free ferromagnetic layer 10;however, there is a limit to stably reduce the product M_(s)·t of themagnetization M_(s) and thickness t of the free ferromagnetic layer 10only through the reduction of the thickness t of the free ferromagneticlayer 10. This is because extremely reducing the thickness t of the freeferromagnetic layer 10 undesirably causes free ferromagnetic layer 10 toexhibit an island-like structure, and to be discontinuous.

[0139] This problem can be solved by forming the free ferromagneticlayer 10 to have a sufficient thickness and modifying a portion of thefree ferromagnetic layer 10 to reduce the magnetization of the modifiedportion. This method achieves the reduction in the effective thicknessand magnetization of the free ferromagnetic layer 10 with the freeferromagnetic layer 10 having a continuous structure, and therebyreduces the product M_(s)·t. This does not causes the decrease in the MRratio, because this does not influence the ferromagnetic properties of acontact portion of the free ferromagnetic layer 10, the contact portionbeing in contact with the tunnel dielectric layer 9. The freeferromagnetic layer 10 is modified so that the modified portion becomesnon-magnetic. Modifying the modified portion to be non-magnetic reducesthe effective thickness of the free ferromagnetic layer 10, and therebyfurther effectively reduces the product M_(s)·t.

[0140] Specifically, the formation of the free ferromagnetic layer 10 soas to exhibit a reduced product M_(s)·t and a continuous structure maybe achieved by methods described below; as shown in FIG. 27, a firstmethod involves forming the free ferromagnetic layer 10 with aferromagnetic layer 31 of ferromagnetic material, and a diffusion layer32 of non-magnetic metal. The ferromagnetic layer 31 is deposited on thetunnel dielectric layer 9, and the diffusion layer 32 is deposited onthe ferromagnetic layer 31; the oxide layer 14 is deposited on thediffusion layer 31. The ferromagnetic layer 9 is typically formed withNiFe. The non-magnetic layer is typically formed with Ru, Pt, Hf, Pd,Al, W, Ti, Cr, Si, Zr, C, Zn, V, Cr, or Mo. Thermal treatment causesdiffusion between the ferromagnetic layer 31 and the diffusion layer 32,and a portion of the ferromagnetic layer 31 is modified to reduce themagnetization M_(s). This allows the free ferromagnetic layer 10 toexhibit a reduced product M_(s)·t and a continuous structure. The freeferromagnetic layer 10 having such structure makes it easy to adjust thedegree of the reduction in the magnetization by the thickness of thediffusion layer 32. Furthermore, this structure stabilizes the productM_(s)·t of the free ferromagnetic layer 10 because the oxide layer 14prevents the top contact layer 15 from absorbing the material includedin the free ferromagnetic layer 10.

[0141] The diffusion layer 32 is not required to be a continuous“layer”. The diffusion layer 32 may be formed to be extremely thin sothat the diffusion layer 32 exhibits an island-like structure.

[0142] The diffusion layer 32 may be disposed in the free ferromagneticlayer 10 or positioned at such an arbitrary position that the diffusionlayer 32 is in contact with the free ferromagnetic layer 10 under theconditions that the diffusion layer 32 is not directly contacted withthe tunnel dielectric layer 9. As shown in FIG. 28, for example, thefree ferromagnetic layer 10 may include ferromagnetic layers 31 and 33,and a diffusion layer 32 disposed therebetween. Disposing the diffusionlayer 32 so as to be in direct contact with the tunnel dielectric layer9 undesirably reduces the MR ratio. A heat treatment causes theinterdiffusion between the diffusion layer 32 and the ferromagneticlayers 31 and 33, and thereby forms the free ferromagnetic layer 10exhibiting a reduced product M_(s)·t and a continuous structure. Itshould be noted that the diffusion layer 32 is not required to be acontinuous “layer” for this case.

[0143] As shown in FIG. 29, a second method involves forming aferromagnetic layer 31 on the tunnel dielectric layer 9 followed byforming a modified layer 34 through nitrizing or oxidizing a surfaceportion thereof. The remainder portion of the ferromagnetic layer 31 andthe modified layer 34 constitutes the free ferromagnetic layer 10.Nitrizing and oxidizing the portion of the ferromagnetic layer 31 may beachieved by subjecting the upper surface of the ferromagnetic layer 31to nitrogen plasma and oxygen plasma, respectively. Nitrizing oroxidizing the portion of the ferromagnetic layer 31 eliminates orreduces the magnetization of the nitrized or oxidized portion, andthereby achieves the formation of the free ferromagnetic layer 10 sothat it exhibits a reduced product M_(s)·t and a continuous structure.The portion of the ferromagnetic layer 31 may be boronized, chlorized,or carbonized instead of nitrized or oxidized.

[0144] For forming the modified layer 34 with oxide, the modified layer34 and the oxide layer 14 are concurrently formed through a methoddescribed below. After the formation of the ferromagnetic layer 31 onthe tunnel dielectric layer 9, a metal film is deposited thereon forforming the oxide layer 14. The upper surface of the metal film issubjected to oxygen plasma. The subjection to the oxygen plasma iscontinued after completing the oxidization of the metal film, andthereby achieves oxidization of a portion of the ferromagnetic layer 31.The thickness of the oxidized portion within the ferromagnetic layer 31can be adjusted by duration of the subjection to the oxygen plasma. Thisis equivalent to the adjustment of the effective thickness of the freeferromagnetic layer 10.

[0145] For the formation of the modified layer 34 with nitride, themodified layer 34 and the oxide layer 14 are concurrently formed througha method described below. After the formation of the ferromagnetic layer31 on the tunnel dielectric layer 9, a metal film is deposited thereonfor forming the oxide layer 14. The upper surface of the metal film issubjected to nitrogen plasma. The subjection to the nitrogen plasma iscontinued after completing the nitrization of the metal film, andthereby achieves nitrization of a portion of the ferromagnetic layer 31.This is followed by the oxidization of the nitrized metal film tocomplete the oxide layer 14. The nitrized portion of the freeferromagnetic layer 10 is not oxidized because of the difference in thereactivity to oxygen.

[0146] The formation of the free ferromagnetic layer 10 so as to exhibita reduced product M_(s)·t and a continuous structure is achieved by astructure shown in FIG. 30. A ferromagnetic layer 31 is formed withferromagnetic material on the tunnel dielectric layer 9. A dopedferromagnetic layer 35 which is formed of the same ferroelectricmaterial as the ferromagnetic layer 31, and doped with non-magneticmetal, is deposited on the ferromagnetic layer 31. The ferromagneticlayer 31 and the doped ferromagnetic layer 35 function as the freeferromagnetic layer 10. The non-magnetic metal is segregated at thegrain boundaries of the ferromagnetic crystals. Doping non-magneticmetal reduces the magnetization of the doped ferromagnetic layer 35, andachieves the formation of the free ferromagnetic layer 10 so as toexhibit a reduced product M_(s)·t and a continuous structure.

[0147] As described below in detail, the technique is useful for thecontrol of the stress-induced magnetic anisotropy of the freeferromagnetic layer 10, which technique controls the composition of thefree ferromagnetic layer 10 to desired values through the suppression ofthe interdiffusion between the top interconnection 3 and the freeferromagnetic layer 10 by disposing the oxide layer 14 (or the nitrideor oxynitride layer). The intensity of the anisotropy field H_(a) isrepresented by the following equation (3):

H _(a)=3(λ·σ)/M _(s),  (3)

[0148] Where λ is the magnetostriction constant of the freeferromagnetic layer, and σ is the stress exerted on the freeferromagnetic layer 10. The magnetostriction constant λ depends on thecomposition of the free ferromagnetic layer 10. The oxide layer 14, onthe other hand, effectively reduces the variations of the compositionsof the free ferromagnetic layers 10 within different magnetoresistanceelements 4, and the variation of the composition of each freeferromagnetic layer 10. This implies that disposing the oxide layer 14enables control of the magnetostriction constant λ of the freeferromagnetic layer 10, and thereby enables control of thestress-induced magnetic anisotropy.

[0149]FIGS. 31 and 32 illustrates an MRAM structure for achievingcontrol of the stress-induced magnetic anisotropy. As shown in FIG. 31,the bottom electrode 2 is formed with metal such as Al, Cu, AlCu, on thesubstrate 1. As shown in FIG. 32, the bottom electrode 2 extends in they-axis direction, while the top interconnection 3 extends in the x-axisdirection. The free ferromagnetic layer 10 has a minor axis in thex-direction and a major axis in the y-direction. This shape provides thefree ferromagnetic layer 10 having a shape-induced magnetic anisotropywith the easy axis in the y-axis direction. As shown in FIG. 31, thecontact layer 12, the oxide layer 13, the seed layer 5, the buffer layer6, and the antiferromagnetic layer 7 are deposited in series over thebottom electrode 2. The bottom contact layer 12 is connected to thebottom electrode 2 through the via contact 11. The fixed ferromagneticlayer 8, the tunnel dielectric layer 9, and the free ferromagnetic layer10 are deposited in series over the antiferromagnetic layer 7. The oxidelayer 14 and the top contact layer 15 are deposited in series over thefree ferromagnetic layer 10, and the top contact layer 15 is connectedto the top interconnection 3 through the via contact 22. The compositionof the free ferromagnetic layer 10 is selected so that themagnetostriction constant λ of the free ferromagnetic layer 10 ispositive. In the case that the free ferromagnetic layer 10 is formedwith Ni_(x)Fe_(1-x), the magnetostriction constant λ is adjusted to apositive value through controlling the parameter x below 0.82.

[0150] As shown in FIG. 33, the aforementioned structure allows thestress-induced and shape-induced magnetization anisotropy to exhibit theeasy axes in the same direction, and thereby stabilizes thecharacteristics of the free ferromagnetic layer 10. The bottominterconnection 2, which extends in the y-axis direction, exerts atensile stress in the x-axis direction (that is, the direction of themajor axis of the free ferromagnetic layer 10), and a compressive stressin the y-axis direction (that is, the direction of the minor axis of thefree ferromagnetic layer 10). It should be noted that the inventors'investigation has depicted that the stress generated by the topinterconnection 3 causes less influences on the free ferromagnetic layer10. Since the magnetostriction constant λ of the free ferromagneticlayer 10 is positive, the compressive stress in the x-axis direction andthe tensile stress in the y-axis direction, which are generated by thebottom interconnection 2, develop the stress-induced magnetic anisotropywith the easy axis in the y-axis direction, and thereby coincide thedirection of the easy axis of the stress-induced magnetic anisotropywith that of the shape-induced magnetic anisotropy. The fact that thestress-induced and shape-induced magnetic anisotropies exhibit the easyaxis in the same direction provides the free ferromagnetic layer 10 withlarge uniaxiality, and thereby allows the free ferromagnetic layer 10 toexhibit a single domain structure. This effectively stabilizes thecharacteristics of the free ferromagnetic layer 10. Specifically, thecoincidence of the directions of the easy axes, resulting from thestress-induced and shape-induced magnetic anisotropy, improves therectangularity of the field magnetization curve of the freeferromagnetic layer 10, and additionally reduces the variation in thecoercive force. An MRAM structure that does not allow the easy axes ofthe stress-induced and shape-induced magnetic anisotropy to be directedin the same direction causes variation in the direction of the easy axisof total anisotropy toward the write interconnection, and therebyundesirably destabilizes the characteristics of the free ferromagneticlayer 10.

[0151] In order to further stabilize the characteristics of the freeferromagnetic layer 10, the free ferromagnetic layer 10 is formed toallow the crystalline magnetic anisotropy thereof to exhibit the easyaxis in the same direction as those of the stress-induced andshape-induced magnetic anisotropy. The coincidence of the easy axis ofthe crystalline magnetic anisotropy with those of the stress-induced andshape-induced magnetic anisotropy enhances the uniaxiality of themagnetic anisotropy of the free ferromagnetic layer 10, and therebystabilizes the characteristics of the free ferromagnetic layer 10.

[0152]FIG. 34 illustrates another MRAM structure for achieving thecontrol of the stress-induced magnetic anisotropy. The bottominterconnection 2 extends in the y-axis direction, while the topinterconnection 3 extends in the x-axis direction. The magnetoresistanceelement 4 is formed so that the free ferromagnetic layer 10 has themajor axis in the x-axis direction and the minor axis in the y-axisdirection. It should be noted that the structure shown in FIG. 34involves that the major and minor axes of the free ferromagnetic layer10 are different by 90° from those shown in FIG. 32. Such shape providesthe free ferromagnetic layer 10 having a shape-induced magneticanisotropy with the easy axis in the x-axis direction. The compositionof the free ferromagnetic layer 10 is selected so that the freeferromagnetic layer 10 has a negative magnetostriction constant λ. Inthe case that the free ferromagnetic layer 10 is formed withNi_(x)Fe_(1-x), the magnetostriction constant λ is adjusted to anegative value through controlling the parameter x over 0.82.

[0153] As is the case of the structure shown in FIGS. 31 and 32, thestructure shown in FIG. 34 coincides the direction of easy axis of thestress-induced magnetic anisotropy (K2) with that of the shape-inducedmagnetic anisotropy (K3), and thereby stabilizes the characteristics ofthe free ferromagnetic layer 10. As described above, the bottominterconnection 2, which extends in the y-axis direction, exerts atensile stress in the x-axis direction (that is, the direction of theminor axis of the free ferromagnetic layer 10), and a compressive stressin the y-axis direction (that is, the direction of the major axis of thefree ferromagnetic layer 10). Since the magnetostriction constant λ ofthe free ferromagnetic layer 10 is negative, the compressive stress inthe x-axis direction and the tensile stress in the y-axis direction,which are generated by the bottom interconnection 2, develop thestress-induced magnetic anisotropy with the easy axis in the x-axisdirection, and thereby coincide the direction of the easy axis of thestress-induced magnetic anisotropy with that of the shape-inducedmagnetic anisotropy.

[0154] In the case that the MRAM is formed to coincide the easy axis ofthe stress-induced magnetic anisotropy with that of the shape-inducedmagnetic anisotropy, as shown in FIG. 35, the free ferromagnetic layer10 is preferably formed so that the stress-induced magnetic anisotropyis larger than the shape-induced magnetic anisotropy. Achieving suchproperties allows the aspect ratio of the free ferromagnetic layer 10(that is, the ratio of the major axis to the minor axis) to be close to1, and thereby reduces the area of the magnetoresistance element 4. Ingeneral, adjusting the aspect ratio of the free ferromagnetic layer 10to a value close to 1.0 weakens the uniaxiality of the magneticanisotropy of the free ferromagnetic layer 10, and thus may cause theformation of closure domains within the free ferromagnetic layer 10. Theformation of closure domains deteriorates the rectangularity of thefield magnetization curve of free ferromagnetic layer 10, and increasesthe variation in the coercive field. Nevertheless, forming the freeferromagnetic layer 10 so as to have the stress-induced magneticanisotropy larger than the shape-induced magnetic anisotropy improvesthe uniaxiality of the free ferromagnetic layer 10, and therebycompensates the decrease in the uniaxiality caused by the reduced aspectratio. Specifically, the technique for adjusting the stress-inducedmagnetic anisotropy to be larger than the shape-induced magneticanisotropy is preferably applied to the free ferromagnetic layer 10having an aspect ratio of 1.0 to 2.0, especially the free ferromagneticlayer 10 having an aspect ratio of 1.25 to 2.0. Adjusting thestress-induced magnetic anisotropy to be larger than the shape-inducedmagnetic anisotropy may be achieved by controlling the magnetostrictionconstant λ to a desired value through appropriately selecting thecomposition of the free ferromagnetic layer 10. Disposing the oxidelayer 10 (or the nitride or oxynitride layer) is of importance becauseit enables an appropriate control of the composition of the freeferromagnetic layer 10.

[0155] The technique for adjusting the stress-induced magneticanisotropy to be larger than the shape-induced magnetic anisotropy isalso preferable because it enables the formation of themagnetoresistance element 4 which is less sensitive to the variation ofthe dimension inevitably caused by the fabrication process of the MRAM.Conventional magnetoresistance elements, which are based on theshape-induced magnetic anisotropy, are sensitive to the dimensionvariation caused by the processes of the elements, (including exposureand etching), and thus experience large variations of the coerciveforces. The technique for adjusting the stress-induced magneticanisotropy to be larger than the shape-induced magnetic anisotropyreduces the influence of the inevitable dimension variation, and therebyeffectively reduces the variation in the coercive force.

[0156] In terms of prevention of the interdiffusion, as shown in FIG. 2,a bottom contact layer 12′ having a sufficiently increased thickness maybe disposed between the magnetoresistance element 4 and the via contact11 in place of the oxide layer 13. This allows the seed layer 5 to beomitted. Correspondingly, as shown in FIG. 3, a top contact layer 15′having a sufficiently increased thickness may be disposed between thefree ferromagnetic layer 10 and the top interconnection 3 in place ofthe oxide layer 13. The bottom and top contact layers 12′ and 15′ aretypically formed with TiN, Ta, Ru, W, Zr, or Mo. The sufficientlyincreased thicknesses of the bottom and top contact layers 12′ and 15′prevent the magnetoresistance element 4 from being diffused withaluminum and copper included in the bottom and top interconnections 2and 3, and also prevent the bottom and top interconnections 2 and 3 frombeing diffused with manganese included in the antiferromagnetic layer 5and nickel included in the buffer layer 6 and the free ferromagneticlayer 10.

[0157] The use of both of the oxide layers 13 and 14, however, ispreferable as shown in FIG. 1. The oxide layers 13 and 14 formed withoxide, which exhibits superior interdiffusion-resistance, are allowed tohave extremely reduced thicknesses. The reduced thicknesses of the oxidelayers 13 and 14 allow the bottom and top interconnections 2 and 3 to bepositioned near the free ferromagnetic layer 10. The arrangement inwhich the bottom and top interconnections 2 and 3 are positioned nearthe free ferromagnetic layer 10 desirably reduces the intensity of thecurrent required for inverting the spontaneous magnetization of the freeferromagnetic layer 10 (that is, the intensity of the current requiredfor achieving write operation).

[0158] Furthermore, although the structure of FIG. 2, including thethick bottom contact layer 12′, prevents the interdiffusion within thebottom interconnection 2, the via contact 11 and the magnetoresistanceelements 4, this structure does not prevent the interdiffusion betweenthe bottom contact layer 12′ and the magnetoresistance element 4.Correspondingly, although the structure of FIG. 3, including the thicktop contact layer 15′, prevents the interdiffusion between the topinterconnection 3 and the magnetoresistance elements 4, this structuredoes not prevent interdiffusion between the top contact layer 15′ andthe magnetoresistance element 4. Therefore, the use of the oxide layer13 is more preferable than that of the thick contact layer 12′, and theuse of the oxide layer 14 is more preferable than that of the thickcontact layer 15′.

[0159] In order to place the bottom interconnection 2 closer to the freeferromagnetic layer 10, as shown in FIG. 4, the oxide layer 13, the seedlayer 5, the buffer layer 6, and the antiferromagnetic layer 7 areformed to substantially entirely cover the upper surface of the bottominterconnection 2. Such structure allows the seed layer 5, the bufferlayer 6, and the antiferromagnetic layer 7 to be incorporated in theinterconnection to which a write current is applied for inverting thespontaneous magnetization of the free ferromagnetic layer 10, andthereby positions the interconnection closer to the free ferromagneticlayer 10.

[0160] As described in the description of the related art, it isundesirable that the tunnel dielectric layer 9 is diffused withmanganese included in the antiferromagnetic layer 7; this deterioratesthe MR ratio of the magnetic tunnel junction. In order to prevent thetunnel dielectric layer 9 from being diffused with manganese included inthe antiferromagnetic layer 7, the fixed ferromagnetic layer 8preferably includes a composite magnetic layer 8 a and a metalferromagnetic layer 8 b. The composite magnetic layer 8 a is disposed onthe antiferromagnetic layer 7, and prevents manganese of theantiferromagnetic layer 7 from being diffused into the tunnel dielectriclayer 9, as described later. The metal ferromagnetic layer 8 b ispreferably formed with metal ferromagnetic alloy including as the mainmaterial cobalt, which has an increased spin polarization ratio, and isthermally stable and hard to be diffused. The use of the metalferromagnetic alloy including cobalt as the main material makes themetal ferromagnetic layer 8 b magnetically hard; “including cobalt asthe main material” means that the material having the maximum atomicpercent out of the materials included in the metal magnetic alloy iscobalt.

[0161] The composite magnetic layer 8 a is formed with a composite thinfilm of mixture including non-oxidized metal ferromagnetic material asmain material and oxide material as sub material, the oxide materialbeing oxide of non-magnetic element more reactive to oxygen than themetal ferromagnetic material. Such composite magnetic layer 8 a exhibitsa structure achieving prevention of the diffusion of manganese withmetallic conductivity and ferromagnetic properties. The metalferromagnetic material used for the composite magnetic layer Ba istypically exemplified by CoFe, and the oxide is exemplified by TaO_(x),HfO_(x), NbO_(x), ZrO_(x), CeO_(x), AlO_(x), MgO_(x), SiO_(x), andTiO_(x). These non-magnetic elements have free energies of oxideformation smaller than those of ferromagnetic elements: Fe, Co, and Ni;therefore these non-magnetic elements are more easily oxidized. Cobaltor metal ferromagnetic alloy including cobalt as main material ispreferably used as the ferromagnetic material of the composite magneticlayer 8 a. Cobalt and the metal ferromagnetic alloy including cobalt asmain material have a high spin polarization ratio and exhibits highoxidization-resistance, while being hard to be diffused because of itsthermally stability.

[0162] It is important that the composite magnetic layer Ba mainlyincludes non-oxidized metal ferromagnetic material for the compositemagnetic layer 8 a to exhibit conductive and ferromagnetic properties.The metallic conductivity of the composite magnetic layer 8 improves theSN ratio of read operations. The ferromagnetic properties of thecomposite magnetic layer 8 a allows exchange interaction from theantiferromagnetic layer 7 to provide the metal ferromagnetic layer 8 b,and thereby allows both of the composite magnetic layer 8 a and themetal ferromagnetic layer 8 b to function as a fixed ferromagneticlayer. In order to avoid the oxidization of the metal ferromagneticmaterial of the composite magnetic layer 8 a, the oxide included in thecomposite magnetic layer 8 a consists of oxide of non-magneticelement(s) more easily oxidized than the metal ferromagnetic material.

[0163] The composite magnetic layer 8 a exhibits either a structureshown in FIG. 6A or shown in FIG. 6B depending on the atomic radiuses ofthe element(s) composing the metal ferromagnetic material, and thenon-magnetic element(s) composing the oxide. When the atomic radius(radii) of the element(s) composing the metal ferromagnetic material islarger than that of the non-magnetic element(s) composing the oxide, asshown in FIG. 6A, the composite magnetic layer 8 a is composed ofcolumnar crystals 31 of the metal ferromagnetic material, and amorphousphase regions 32 which consists of mixture of the metal ferromagneticmaterial and oxide of the non-magnetic element(s). The reason why thecomposite magnetic layer 8 a exhibits such structure may be that thenon-magnetic element(s), which has a large atomic radius, inhibitscrystallization of the metal ferromagnetic material. In the case thatCoFe is used as the metal ferromagnetic material of the compositemagnetic layer Ba, the composite magnetic layer 8 a exhibits thestructure shown in FIG. 6A when TaO_(x), HfO_(x), NbO_(x), or CeO_(x) isused as the oxide of the composite magnetic layer 8 a.

[0164] When the atomic radius (radii) of the element(s) composing themetal ferromagnetic material is smaller than that of the non-magneticelement(s) composing the oxide, as shown in FIG. 6B, the compositemagnetic layer Ba is composed of granular crystals 33 of the metalferromagnetic material, and amorphous oxide 34 which is formed throughsegregation of the oxide at the grain boundaries of the granularcrystals 33. Material exhibiting such structure is sometimes called asgranular alloy. It should be noted that the granular crystals 33 are nottotally isolated from one another; some of the granular crystals 33 arein direct contact with other adjacent granular crystals 33, or throughpinholes disposed through the non-magnetic oxide 34. Such structureallows the composite magnetic layer 8 a to exhibit soft ferromagnetismbecause of the magnetic coupling among the granular crystals 33, andalso to exhibit metallic conductivity. In the case that CoFe is used asthe metal ferromagnetic material of the composite magnetic layer 8 a,the composite magnetic layer 8 a exhibits the structure shown in FIG. 6Bwhen AlO_(x), MgO_(x), SiO_(x), or TiO_(x) is used as the oxide.

[0165] For both of the structures shown in FIGS. 6A and 6B, thecomposite magnetic layer 8 a exhibits the structure that prevents thediffusion using fineness of the non-magnetic oxide included therein.Additionally, the composite magnetic layer 8 a, which includes oxide,functions as a trap for manganese, which is reactive to oxygen. Whenmanganese is diffused into the composite magnetic layer 8 a, thediffused manganese is stabilized through reaction to oxygen, and trappedin the composite magnetic layer 8 a. Additionally, the compositemagnetic layer 8 a exhibits superior diffusion-resistance because it isalmost free from the grain boundaries in contrast to usual metalferromagnetic layers, and thus the high-speed diffusion route iseliminated. Because of these effects, the composite magnetic layer 8 aenjoys diffusion-resistance for achieving effective suppression of thediffusion of manganese into the tunnel dielectric layer 9 withoutblocking the magnetic and electric coupling within the fixedferromagnetic layer. Such characteristics cannot be obtained withconventional oxide diffusion barrier layers.

[0166] The composite magnetic layer 8 a may be formed through a reactivesputtering technique with sputtering gas including oxygen gas. Gasmixture of oxygen gas and argon gas is typically used as the sputteringgas. An alloy target including metal ferromagnetic material andnon-magnetic element(s) more easily oxidized than the metalferromagnetic material is typically used as the sputtering target.Sputtering the alloy target with sputtering gas including oxygen gascauses oxygen to react the non-magnetic metal in preference to the metalferromagnetic material. Therefore, appropriately adjusting thecomposition of oxygen of the sputtering gas achieves the formation ofthe composite magnetic layer 8 a so that only the non-magnetic metal isoxidized without oxidizing the metal ferromagnetic material.

[0167]FIG. 7 is a graph illustrating resistivities of thin filmsobtained through sputtering a (Co₉₀Fe₁₉)Ta₁₅ alloy target, whichconsists of ferromagnetic CoFe and non-magnetic Ta, with sputtering gasincluding oxygen gas, and FIG. 8 is a graph illustrating saturatedmagnetizations of these thin films. The horizontal axes of these graphsrepresent ratio [O₂]/[Ar], where [O₂] designates a flow rate of oxygengas introduced to the sputtering chamber in the unit of sccm, and [Ar]designates a flow rate of argon gas in the unit of sccm. The formed thinfilms exhibits the structure shown in FIG. 6A, that is, the structuremostly including amorphous oxide layer having a composition representedby the composition formula of (CoFe)_(x)Ta_(1-z)O_(x), and partiallyincluding columnar CoFe crystals. For reduced [O₂]/[Ar] ratios, as shownin FIGS. 7 and 8, these thin films exhibits metallic conductivity, andlarge saturated magnetization. Increase in the [O₂]/[Ar] ratio above 0.2remarkably increases the resistivities of the thin films and decreasesthe saturated magnetizations.

[0168] These graphs proves that reduction in the [O₂]/[Ar] ratio below0.2 is required for allowing CoFe of these thin films to exist in themetallic state. This fact is ensured by an XPS (X-ray photoelectronspectroscopy) analysis. FIG. 9 illustrates a Co_(2p) spectrum obtainedby the XPS analysis with respect to the thin films with the [O₂]/[Ar]ratios adjusted to 0.13 and 0.54, respectively. The Co₂, spectrumillustrated in FIG. 9 depicts that 70 percents of cobalt within the thinfilm is metallic for the [O₂]/[Ar] ratio of 0.13, while cobalt withinthe thin film is oxidized for the [O₂]/[Ar] ratio of 0.54.

[0169] In the case that CoFe is used as the metal ferromagneticmaterial, and any of TaO_(x), HfO_(x), NbO_(x), ZrO_(x), AlO_(x),MgO_(x), and SiO_(x) is used as the oxide of the non-magnetic metal,adjusting the [O₂]/[Ar] ratio below 0.2 enables the formation of thecomposite metallic layer 8 a with CoFe remaining metallic.

[0170] The aforementioned composite thin film, which includesnon-oxidized metal ferromagnetic material as main material and oxide ofnon-magnetic element(s) more reactive to oxygen than the metalferromagnetic material as sub material, is relatively magnetically softbecause it fails to exhibit a crystalline structure observed in metalferromagnetic layers, including size-reduced ferromagnetic crystallinegrains, and exhibiting reduced crystalline magnetic anisotropy. Thischaracteristics can be used for excluding nickel from the freeferromagnetic layer 10. The diffusion of nickel into the tunneldielectric layer 9 causes decrease in the MR ratio of themagnetoresistance element 4. Additionally, the diffusion of nickel intothe top interconnection 3 increases the resistance of the topinterconnection 3. In terms of nickel diffusion, nickel is preferablyexcluded from the free ferromagnetic layer 10.

[0171]FIG. 10 shows a structure for excluding nickel from the freeferromagnetic layer 10 with such composite thin film. The freeferromagnetic layer 10 is composed of a metal ferromagnetic layer 10formed of material having an increased spin polarization with highthermal stability and diffusion-resistance, and a composite magneticlayer 10 formed of the aforementioned composite thin film. The metalferromagnetic layer 10 a is disposed on the tunnel dielectric layer 9,and the composite magnetic layer 10 b is disposed on the metalferromagnetic layer 10 a. The metal ferromagnetic layer 10 a ispreferably formed with ferromagnetic material including cobalt as mainmaterial, typically formed with CoFe. The composite magnetic layer 10 bis formed of a composite thin film including mixture of nickel-freeferromagnetic material (typically CoFe), and non-magnetic metal oxide.The free ferromagnetic layer 10 structured as thus described achieveshigh MR ratio because of the direct contact of the metal ferromagneticlayer 10 a, which has an increased spin polarization and thermalstability, with the tunnel dielectric layer 9. Additionally, themagnetically soft composite magnetic layer 10 b makes the metalferromagnetic layer 10 a magnetically soft through effecting exchangeinteraction on the metal ferromagnetic layer 10 a, and thereby causesthe whole of the free ferromagnetic layer 10 to be magnetically soft.

[0172] In order to obtain a free ferromagnetic layer magnetically softerthan the free ferromagnetic layer shown in FIG. 10, a softerferromagnetic layer 10 c of nickel-including ferromagnetic material,typically NiFe, may be formed on the composite magnetic layer 10 b asshown in FIG. 11. The composite magnetic layer 10 b, which is formedwith the aforementioned composite thin film, exhibitsdiffusion-resistance against nickel. Therefore, diffusion of nickelincluded in the soft ferromagnetic layer 10 c into the tunnel dielectriclayer 9 is prevented by the composite magnetic layer 10 b. Additionally,nickel diffusion from the soft magnetic layer 10 c to the top electrodelayer 3 is prevented by the aforementioned oxide layer 14. The structureshown in FIG. 11 is preferable in terms of prevention of nickeldiffusion into the tunnel dielectric layer 9 and the top electrode layer3 with an improved MR ratio.

[0173] The magnetoresistance elements 4 shown in FIGS. 10 and 11 sufferfrom a problem of an increased demagnetizing field caused by an increasein the total saturated magnetization of the free ferromagnetic layer 10,which is caused by incorporating the composite magnetic layer 10 b inthe free ferromagnetic layer 10. In order to reduce the demagnetizingfield, as shown in FIG. 12, the free ferromagnetic layer 10 ispreferably composed of a metal ferromagnetic layer 10 d, a compositemagnetic layer 10 e, a non-magnetic layer 10 f, a composite magneticlayer 10 g, and a soft ferromagnetic layer 10 h. The metal ferromagneticlayer 10 d is formed with ferromagnetic material having a large spinpolarization ratio, such as CoFe. The composite magnetic layers 10 e and10 g are formed with the aforementioned composite thin film to exhibitsoft ferromagnetism. The soft ferromagnetic layer 10 h is formed withnickel-including ferromagnetic material, typically NiFe. Thenon-magnetic layer 10 f is formed with material that provides strongantiferromagnetical coupling between the composite magnetic layers 10 eand 10 g, typically Cu, Cr, Rh, Ru, or RuO_(x).

[0174] The non-magnetic layer 10 f antiferromagnetically couplesspontaneous magnetization of the metal ferromagnetic layer 10 d and thecomposite magnetic layer 10 e with that of the composite magnetic layer10 g and the soft ferromagnetic layer 10 h to be antiparallel thereto.As disclosed in U.S. Pat. No. 5,408,377, forming the free ferromagneticlayer 4 with two ferromagnetic layers and non-magnetic layer whichantiferromagnetically couples the ferromagnetic layers reduces theeffective saturated magnetization of the free ferromagnetic layer 4, andthus reduces the demagnetization field. The reduced demagnetizationfield reduces the coercive field of the free ferromagnetic layer 4.Accordingly, the structure shown in FIG. 12 increases the MR ratio,prevents nickel diffusion into the tunnel dielectric layer 9 and the topelectrode layer 3, and makes the free ferromagnetic layer 10magnetically soft.

[0175] In the case that the free ferromagnetic layer 10 is sufficientlysoft, the structure shown in FIG. 12 may fail to include the softferromagnetic layer 10 h. Omitting the soft ferromagnetic layer 10 hexcludes nickel from the free ferromagnetic layer 10, and therebyfundamentally eliminates the undesirable influence caused by nickeldiffusion.

[0176] As shown in FIG. 13, in order to provide enhancedantiferromagnetic coupling within the free ferromagnetic layer 10, thestructure shown in FIG. 12 preferably includes metal ferromagneticlayers 10 i on the both surfaces of the non-magnetic layer 10 f. Formingthe metal ferromagnetic layers 10 i with nickel-free ferromagnetic alloyincluding cobalt as main material enhances the antiferromagneticcoupling. It is preferable that such metal ferromagnetic layers 10 i.The metal ferromagnetic layers 10 i preferably formed with Co orCo₉₀Fe₁₀.

[0177] In order to improve the magnetoresistance element, as shown inFIG. 14, a magnetic bias layer 20 may be provided for the freeferromagnetic layer 10. Appling an appropriate bias magnetic fieldeliminates asymmetricity in the hysteresis curve of the freeferromagnetic layer 10 with respect to magnetic field directions.

[0178] The magnetic bias layer 20 includes a protective layer 16, aferromagnetic layer 17 of ferromagnetic material, an antiferromagneticlayer 18 of antiferromagnetic material, and a protective layer 19. Theprotective layers 16 and 19 are typically formed with Ta or Zr. Theferromagnetic layer 17 is typically formed with CoFe. Theantiferromagnetic layer 18 is formed with manganese-includingantiferromagnetic material, typically PtMn, or IrMn. The positions ofthe ferromagnetic layer 17 and the antiferromagnetic layer 18 may bepermutated.

[0179] As is the case of the antiferromagnetic layer 7, theantiferromagnetic layer 18 potentially causes manganese diffusion. Inorder to resolve the problem of the manganese diffusion, in the casethat the magnetoresistance element includes the magnetic bias layer 20,the magnetic bias layer 20 is disposed on the oxide layer 14 and anoxide layer 21 is disposed between the magnetic bias layer 20 and thetop contact layer 15. The oxide layer 14 prevents manganese diffusioninto the tunnel dielectric layer 9, while the oxide layer 21 preventsmanganese diffusion into the top interconnection 3.

[0180] Characteristics required for the oxide layer 21 are identical tothose for the oxide layer 13, and thus preferred materials andstructures for the oxide layer 13 are also preferable for the oxidelayer 21. Firstly, in order for the oxide layer 21 to befine-structured, thermally stable against high temperature, and toexhibit an increased resistance against the interdiffusion, the oxidelayer 21 is preferably formed with oxide of aluminum, magnesium,silicon, hafnium, lithium, calcium, or titanium. Furthermore, theresistance of the oxide layer 21 in the thickness direction ispreferably smaller than that of the tunnel dielectric layer 9.Additionally, the thickness of the oxide layer 21 is preferably reducedbelow 1 nm, to extremely reduce the resistance of the oxide layer 14,and to thereby improve the SN ratio of detecting the resistance of thetunnel dielectric layer 9. Finally, forming the oxide layer 21 with thesame material as the tunnel dielectric layer 9 desirably allows theformation of the oxide layer 21 and the tunnel dielectric layer 9 withthe same apparatus and material, and thereby reduces the fabricationcost of the MRAM.

[0181] The oxide layer 21 is preferably formed with oxide of materialmore reactive to oxygen than material included in layers in contact withthe bottom and top surfaces thereof (that is, the bottom contact layer12 and the seed layer 5), the bottom surface designating the surface onthe side of the substrate 1. The use of oxide of material not satisfyingthis requirement undesirably allows the oxide diffusion into the layersin contact with the bottom and top surfaces of the oxide layer 21, andthus spoils the diffusion resistance of the oxide layer 21. In the casethat tantalum is used for the top contact layer 15, and tantalum orchromium is used for the protective layer 19, the oxide layer 21 ispreferably formed with oxide of aluminum, magnesium, silicon, hafnium,lithium, calcium, or tantalum, which have smaller free energies of oxideformation than those of tantalum and chromium.

[0182] The structures of the bottom contact layer 12′ shown in FIG. 2,the top contact layer 15′ shown in FIG. 3, the fixed ferromagnetic layer8 shown in FIG. 5, the free ferromagnetic layers 10 shown in FIGS. 10through 14, the magnetic bias layer 20 shown in FIG. 14 may be combined.For example, a structure shown in FIG. 15 may be the case. The MRAMshown in FIG. 15 has the fixed ferromagnetic layer 8 composed of thecomposite magnetic layer 8 a and the ferromagnetic layer 8 b.Additionally, the MRAM has the free ferromagnetic layer 10 composed ofthe metal ferromagnetic layer 10 a, the composite magnetic layer 10 band the soft ferromagnetic layer 10 c. Furthermore, the magnetic biaslayer 20 is disposed on the oxide layer 14, and the oxide layer 21 isdisposed between the magnetic bias layer 20 and the top contact layer15.

[0183]FIG. 16 shows a more practical structure of a magnetoresistanceelement. The top contact layer 15 and the top interconnection 3 isconnected through a via contact 22. The via contact 22 is typicallyformed with Al, Cu, W, or TiN. Additionally in the magnetoresistanceelement shown in FIG. 16, the tunnel dielectric layer 9 is positioned soas not to overlap the via contact 11 in the direction perpendicular tothe major surface of the substrate 1. Such arrangement of the tunneldielectric layer 9 desirably reduces defections of the tunnel dielectriclayer 9. The via contact 11, which is formed with metal, inevitably hasan uneven surface thereon. Positioning the tunnel dielectric layer 9 soas to overlap the via contact 11 as shown in FIG. 1 develops an unevensurface thereon, and thus tends to generate defects within the tunneldielectric layer 9. The arrangement in which the tunnel dielectric layer9 does not overlap the via contact 11, as shown in FIG. 16, reducesdefects in the tunnel dielectric layer 9.

[0184] In the case that such arrangement is adopted that tunneldielectric layer 9 does not overlap the via contact 11, the structuresof the bottom contact layer 12′ shown in FIG. 2, the top contact layer15′ shown in FIG. 3, the fixed ferromagnetic layer 8 shown in FIG. 5,the free ferromagnetic layers 10 shown in FIGS. 10 through 14, themagnetic bias layer 20 shown in FIG. 14 may be also combined. Forexample, the structure shown in FIG. 17 may be the case. In an MRAMshown in FIG. 17, the magnetoresistance element 4 and the bottominterconnection 2 are electrically connected through the thick bottomcontact layer 12′. Furthermore, the fixed ferromagnetic layer 8 iscomposed of the compound magnetic layer 8 a and the ferromagnetic layer8 b. The magnetic bias layer 20 is disposed on the oxide layer 14, whilethe oxide layer 21 is disposed between the magnetic bias layer 20 andthe top contact layer 15.

[0185] Such arrangement that the tunnel dielectric layer 9 does notoverlap the via contact 11 in the direction perpendicular to the majorsurface of the substrate 1, as shown in FIG. 18, is preferable for thecase that the bottom interconnection 2, which is electrically connectedto the via contact 11, is dedicated to read operations of the MRAM, anda write interconnection 2′ is additionally disposed parallel to theinterconnection 2; the write interconnection 2′ is electrically isolatedfrom the bottom interconnection 2. When this arrangement is adopted,determining the data stored in the free ferromagnetic layer 10 (that is,detecting the resistance of the tunnel dielectric layer 9) is achievedthrough detecting a current generated by a voltage applied between thetop and bottom interconnections 3 and 2. The data write into the freeferromagnetic layer 10, on the other hand, is achieved by developingcurrents through the top interconnection 3 and the write interconnection2′.

[0186] Furthermore, in the case that the top interconnection 3 is formedwith a copper layer, as shown in FIG. 18, the oxide layer 14, which isdisposed between the free ferromagnetic layer 10 and the topinterconnection 3, is disposed to separate the top interconnection 3from a interlayer dielectric (not denoted by numeral) covering themagnetoresistance element 4. In this case, the top contact layer 15 isomitted; instead, a protective layer 23 is disposed between the freeferromagnetic layer 10 and the oxide layer 14. The protective layer 23is typically formed with tantalum or zirconium. This structure desirablyprevents both the interdiffusion between the magnetoresistance element 4and the top interconnection 3 and the diffusion of the interlayerdielectric with copper included in the top interconnection 3.

[0187] It is apparent that the structures of the magnetoresistanceelements 4 described in this embodiment may be applied to magnetic readheads.

EXPERIMENTAL RESULTS

[0188] The improvement in the magnetoresistance element throughdisposing the oxide layers 13 and. 14 has been investigated using twosamples having the structures described below:

Comparative Example 1

[0189] substrate/Ta(3 nm)/AlCu(50 nm)/Ta(3 nm)/Ni₈₁Fe₁₉(3 nm)/IrMn(10nm)/Co₉₀Fe₁₀(6 nm)/AlO_(x)(2 nm)/Co₉₀Fe₁₀(2.5 nm)/Ni₈₁Fe₁₉(7.5 nm)/Ta(5nm)/AlCu(300 nm), and

Example 1 (The Present Invention)

[0190] substrate/Ta(3 nm)/AlCu(50 nm)/Ta(3 nm)/Al₂O₃(1 nm)/Ta(3nm)/Ni₈₁Fe₁₉(3 nm)/IrMn(10 nm)/Co₉₀Fe₁₀(6 nm)/AlO_(x)(2 nm)/Co₉₀Fe₁₀(2.5nm)/Ni₈₁Fe₁₉(7.5 nm)/Al₂O₃(1 nm)/Ta(5 nm)/AlCu(300 nm)

[0191] Comparative Example 1 corresponds with the structure shown inFIG. 1 with the oxide layers 13 and 14 omitted. Example 1 correspondswith the structure shown in FIG. 1, having the oxide layers 13 and 14formed with Al₂O₃ of 1 nm in thickness. The AlCu layer of 50 nm inthickness corresponds with the bottom interconnection 2, while the AlCulayer of 300 nm in thickness corresponds with the top interconnection 3.

[0192]FIG. 19 illustrates changes in MR ratios of Comparative Example 1and Example 1 depending on the thermal treatment temperature.Comparative Example 1 experience reduction in the MR ratio after thethermal treatment at the temperatures over 300° C., while Example 1 isresistant to the thermal treatment at or below 300° C., and experiencesminor reduction after the thermal treatment over 300° C. It isconsidered that this results from that the oxide layers 13 and 14,formed with Al₂O₃ of 1 nm in thickness, prevents the diffusion ofaluminum and copper form the AlCu layer into the magnetic tunneljunction.

[0193] Next, the effect of the oxide layer 13 against the diffusion ofnickel and manganese from the buffer layer 6 and the antiferromagneticlayer 7 into the bottom interconnection 2 has been investigated usingthree samples having the structures described below:

Comparative Example 2

[0194] substrate/Ta(3 nm)/AlCu(50 nm)/Ta(3 nm)/Ni₈₁Fe₁₉(3 nm)/IrMn(10nm)/Co₉₀Fe₁₀(4 nm)

Example 2 (The Present Invention)

[0195] substrate/Ta(3 nm)/AlCu(50 nm)/Ta(3 nm)/Al₂O₃(1 nm)/Ta(3nm)/Ni₈₁Fe₁₉(3 nm)/IrMn(10 nm)/Co₉₀Fe₁₀(4 nm), and

Example 3 (The Present Invention)

[0196] substrate/Ta(3 nm)/AlCu(50 nm)/Ta(3 nm)/MgO(1 nm)/Ta(3nm)/Ni₈₁Fe₁₉(3 nm)/IrMn(10 nm)/Co₉₀Fe₁₀(4 nm)

[0197] Comparative Example 2 corresponds with a portion of the structureof FIG. 1 between the tunnel dielectric layer 9 and the substrate 1 withthe oxide layer 13 omitted. Example 2 corresponds a portion of thestructure of FIG. 1 between the tunnel dielectric layer 9 and thesubstrate 1, having the oxide layer 13 formed with Al₂O₃ of 1 nm inthickness. Example 3 corresponds a portion of the structure of FIG. 1between the tunnel dielectric layer 9 and the substrate 1, having theoxide layer 13 formed with MgO of 1 nm in thickness. For all thesamples, the AlCu layers of 50 nm in thickness correspond with thebottom electrodes 2.

[0198]FIG. 20 illustrates changes in sheet resistances of the AlCulayers depending on the thermal treatment temperature obtained fromComparative Example 2 and Example 2. Comparative Example 2, which doesnot include the oxide layer 13, suffers from a remarkable increase inthe sheet resistance of the AlCu layer after the thermal treatment atthe temperatures over 300° C. In contrast, Example 2, which includes theoxide layer 13 of Al₂O₃, exhibits no increase in the sheet resistanceafter the thermal treatment at 350° C., and experiences only a minorincrease in the sheet resistance after the thermal treatment at about400° C.

[0199]FIG. 21 illustrates sheet resistances of the AlCu layers ofComparative Example 2, and Example 2 and 3 after thermal treatment atvarious temperatures. As shown in FIG. 21, Comparative Example 2, whichdoes not include the oxide layer 13, experiences an increase in thesheet resistance after the thermal treatment at the temperatures of 350°and 400° C. In contrast, Example 2, which has the oxide layer 13 formedwith Al₂O₃, and Example 3, which has the oxide layer 13 formed with MgO,are not influenced by the thermal treatment at 350° C. Furthermore,Examples 2 and 3 experience only minor increases in the sheet resistanceafter the thermal treatment at 400° C.

[0200] Next, the effect of the oxide layer 14 against the interdiffusionbetween the top interconnection 3 and the free ferromagnetic layer 10has been investigated using two samples having the following structures:

Comparative Example 3

[0201] substrate/Ta(1.5 nm)/Co₉₀Fe₁₀(10 nm)/AlO_(x)(2 nm)/Co₉₀Fe₁₀(2.5nm)/Ni₈₁Fe₁₉(7.5 nm)/Ta(5 nm)/AlCu(300 nm), and

Example 4 (The Present Invention)

[0202] substrate/Ta(1.5 nm)/Co₉₀Fe₁₀(10 nm)/AlO_(x)(2 nm)/Co₉₀Fe₁₀(2.5nm)/Ni₈₁Fe₁₉(7.5 nm)/Al₂O₃(1 nm)/Ta(5 nm)/AlCu(300 nm).

[0203] For both of Comparative Example 3 and Example 4, the Co₉₀Fe₁₀layer of 2.5 nm in thickness and the Ni₈₁Fe₁₉ layer of 7.5 nm inthickness correspond with the free ferromagnetic layer 10, while theAlCu layer of 300 nm in thickness corresponds with the topinterconnection 3. The Al₂O₃ layer incorporated within the Example 4corresponds with the oxide layer 14. In order to exclude influencescaused by manganese, Comparative Example 3 and Example 4 do not includethe antiferromagnetic layer 7.

[0204]FIG. 22 illustrates changes in the MR ratios of ComparativeExample 3 and Example 4 depending on the thermal treatment temperature.Example 3 suffers from decrease in the MR ratio after the thermaltreatment at or over 300° C., while Example 4 is resistant against thethermal treatment at relatively high temperature up to 370° C.Furthermore, Example 4 achieves an MR ratio of 20% after the thermaltreatment at 400° C. It is considered that this results from that theoxide layer 14, formed with Al₂O₃ of 1 nm in thickness prevents thediffusion of aluminum and copper from the AlCu layer into the magnetictunnel junction.

[0205]FIG. 23 illustrates a magnetization curve of the Ni₈₁Fe₁₉ layerwithin Comparative Example 3, while FIG. 24 illustrates a magnetizationcurve of the N₈₁Fe₁₉ layer within Example 4. The magnetization curvesare obtained with a vibrating magnetometer. As shown in FIG. 23, Example3 experiences a decrease in the saturated magnetization, deteriorationin the rectangularity of the hysteresis curve, and an increase in thecoercive field after the thermal treatment at 380° C. This results fromthe interdiffusion between the AlCu layer and the Ni₈₁Fe₁₉ layer. Incontrast, the magnetization curve of Example 4 is not influenced by thethermal treatment at 380° C. This results from that the Al₂O preventsthe interdiffusion between the AlCu layer and the Ni₈₁Fe₁₉ layer.

[0206] Next, the effect of the oxide layer 14 formed with Al₂O₃ or MgOagainst the interdiffusion between the top interconnection 3 and thefree ferromagnetic layer 10 has been investigated with four samplesdescribed below:

Comparative Example 4

[0207] substrate/Co₉₀Fe₁₀(2.5 nm)/Ni₈₁Fe₁₉(7.5 nm)/Ta(6 nm)/AlCu(300nm),

Comparative Example 5

[0208] substrate/Co₉₀Fe₁₀(2.5 nm)/Ni₈₁Fe₁₉(7.5 nm)/Ta(50 nm)/AlCu(300nm),

Example 5 (The Present Invention)

[0209] substrate/Co₉₀Fe₁₀(2.5 nm)/Ni₈₁Fe₁₉(7.5 nm)/Al₂O₃(1 nm)/Ta(6nm)/AlCu(300 nm), and

Example 6 (The Present Invention)

[0210] substrate/Co₉₀Fe₁₀(2.5 nm)/Ni₈₁Fe₁₉(7.5 nm)/MgO(1 nm)/Ta(6nm)/AlCu(300 nm).

[0211] Comparative Example 4 corresponds a portion of the structure ofFIG. 1 with the oxide layer 14 omitted, the portion being positionedover the tunnel dielectric layer 9 apart from the substrate 1.Comparative Example 5 is similar to Comparative Example 4 in thestructure except for that the Ta layer, which corresponds with the topcontact layer 15′, is increased in thickness up to 50 nm, to preventsthe diffusion from AlCu layer, which corresponds with the topinterconnection 3. Example 5 corresponds with a portion of the structureshown in FIG. 1, the portion being positioned over the tunnel dielectriclayer 9 apart from the substrate 1, with the oxide layer 14 formed withAl₂O₃ of 1 nm in thickness. Example 6 corresponds with a portion of thestructure shown in FIG. 1, the portion being positioned over the tunneldielectric layer 9 apart from the substrate 1, with the oxide layer 14formed with MgO of 1 nm in thickness. For all of Comparative Examples 4and 5, and Examples 5 and 6, the layered structure consisting of theCo₉₀Fe₁₀ layer and the Ni₈₁Fe₁₉ layer corresponds with the freeferromagnetic layer.

[0212]FIG. 25 illustrates changes in saturated magnetizations of thefree ferromagnetic layers within Comparative Examples 4 and 5, andExamples 5 and 6 depending on the thermal treatment temperature. Asshown in FIG. 25, Comparative Example 4 experiences reduction in thesaturated magnetization and increase in the coercive force after thethermal treatment at 380° C.; the thermal treatment at 400° C.eliminates the saturated magnetization, and results in that themagnetization curve is resorted to exhibit paramagnetic properties.Although not exhibiting as severe increase of the coercive force asComparative Example 4, Comparative Example 5 experiences decrease in thesaturated magnetization as increase in the thermal treatmenttemperature. Furthermore, Comparative Example 4 experiences severeinterdiffusion between the AlCu layer and the Co₉₀Fe₁₀/Ni₈₁Fe₉₁ layers,where the AlCu layer corresponds to the top interconnection, and theCo₉₀Fe₁₀/Ni₈₁Fe₉₁ layers correspond to the free ferromagnetic layer 10.Although preventing the interdiffusion between the Al layer and thelayered structure of the Co₉₀Fe₁₀ layer, and the Ni₈₁Fe₁₉ layer,Comparative Example 5 exhibits the interdiffusion between the Ta layerof 50 nm in thickness and the structure consisting of the Al layer, theCo₉₀Fe₁₀ layer and the Ni₈₁Fe₁₉ layer, where the Ta layer correspondswith the top contact layer 151. In contrast, Examples 5 and 6 do notexhibit changes in the saturated magnetizations for all the testedthermal treatment temperatures; this depicts that Examples 5 and 6 areresistant against the thermal treatment at relatively high temperaturesup to 400° C.

[0213] Next, it has been investigated that the oxide layer 14 formedwith AlO_(x) layer achieves reduction in the thickness of the freeferromagnetic layer 10 down to or below 3 nm, using samples describedbelow:

Example 7 (The Present Invention)

[0214] substrate/Ta(10 nm)/AlO_(x)/Ni₈₁Fe₁₉/AlO_(x)/Ta(10 nm).

[0215] The AlO_(x) layer relatively close to the substrate correspondswith the tunnel dielectric layer 9, and the Ni₉₁Fe₁₉ layer correspondswith the free ferromagnetic layer 10. Additionally, the AlO_(x) layerrelatively far from the substrate corresponds with the oxide layer 14,and the tantalum layer far from the substrate corresponds with the topcontact layer 15. The AlO_(x) layer corresponding with the tunneldielectric layer 9 is formed through oxidization of an aluminum film of1.5 nm in thickness with oxygen plasma, while the AlO_(x) layercorresponding with the oxide layer 14 is formed through oxidization ofan aluminum film of 0.65 nm in thickness with oxygen plasma. Thethicknesses of the Ni₈₁Fe₁₉ layers are selected out of 3.0 nm, 2.6 nm,2.2 nm, 1.4 nm and 1.0 nm. The Ni₈₁Fe₁₉ layers are formed throughsputtering. The thermal treatment and measurement of magnetizations areimplemented under the same conditions used for the samples in connectionwith FIG. 26. That is, the thermal treatment temperature ranges between250° and 400° C., and the duration is 30 minutes. The magnetizationsM_(s) is measured with a vibrating magnetometer.

[0216] As shown in FIG. 36, Example 7 exhibits stable 4πM_(s)·t againstthe thermal treatment up to 400° C. even if the thickness thereof isreduced down to 1.4 nm. Example 7 achieves reduction in 4πM_(s)·t downto 1.2 (T·nm) from the conventional value 2.2 (T·nm). The coercive forceof the free ferromagnetic layers within Example 7 is stable, rangingbetween 0.5 and 1.5 (Oe). This implies that disposing the oxide layer 14achieves the reduction in the free ferromagnetic layer 14, and therebyreduces the coercive force sufficiently and stably.

[0217] Next, the effect of the reduction in the product M_(s)·t, whichis allowed by forming the free ferromagnetic layer 10 with theferromagnetic layer 10 a and the diffusion layer 10 b, has beeninvestigated with samples described below: Example 8 (The PresentInvention)

[0218] substrate/Ta(10 nm)/AlOx/Ni₈₁Fe₁₉(1.6 nm)/non-magnetic metallayer/AlOx/Ta(10 nm).

[0219] The AlO_(x) film relatively close to the substrate correspondswith the tunnel dielectric layer 9. The Ni₈₁Fe₁₉ film corresponds withthe ferromagnetic layer 10 a within the free ferromagnetic layer 10, andthe non-magnetic metal layer corresponds with the diffusion layer 10 b.The AlO_(x) film relatively far from the substrate corresponds with theoxide layer 14, and the Ta film corresponds with the top contact layer15. The AlO_(x) film corresponding with the tunnel dielectric layer 9 isformed through oxidization of an aluminum film of 1.5 nm in thicknesswith oxygen plasma, while the AlO_(x) film corresponding with the oxidelayer 14 is formed through oxidization of an aluminum film of 0.65 nm inthickness with oxygen plasma. A tantalum films having thickness of 0.6and 0.0.3 nm, a ruthenium film having a thickness of 0.3 nm, and acopper film having a thickness of 0.3 nm are used as the non-magneticmetal layer.

[0220] As illustrated in FIG. 37, the structure of Example 8 achievesreduction in 4πM_(s)·t of the Ni₈₁Fe₁₉ film down to or below 1 (T·nm).Additionally, 4πM_(s)·t of the Ni₈₁Fe₁₉ film within Example 8 is stableagainst the thermal treatment at 400° C. The coercive forces of theNi₈₁Fe₁₉ film of Example 8 are reduced below 1.5 (Oe); this implies thatExample 8 achieves the sufficiently reduced coercive force.

[0221] Next, it has been investigated that disposing a non-magneticmetal layer in the free ferromagnetic layer 10 achieves the reduction inthe product M_(s)·t using samples described below:

Example 9 (The Present Invention)

[0222] substrate/Ta(10 nm)/AlO_(x)/Ni₈₁Fe₁₉(1.6 nm)/non-magnetic metallayer/Ni₈₁Fe₁₉(0.8 nm)/AlO_(x)/Ta(10 nm).

[0223] The AlO_(x) film relatively close to the substrate correspondswith the tunnel dielectric layer 9, and the pair of the two Ni₈₁Fe₁₉films, which sandwiches the non-magnetic metal layer, corresponds withthe ferromagnetic layers 10 a and 10 c, respectively. The non-magneticmetal layer corresponds with the diffusion layer 10 b. Additionally, theAlO_(x) film relatively apart from the substrate corresponds with theoxide layer 14, and the Ta film relatively apart from the substratecorresponds with the top contact layer 15. The AlO_(x) filmcorresponding with the tunnel dielectric layer 9 is formed throughoxidization of an aluminum film of 1.5 nm in thickness with oxygenplasma, and the AlO_(x) film corresponding with the oxide layer 14 isformed through oxidization of an aluminum film of 0.65 nm in thicknesswith oxygen plasma. A tantalum film of 0.3 nm is used as thenon-magnetic metal film.

[0224] As illustrated in FIG. 37, Example 9 achieves reduction in4πM_(s)·t of the free ferromagnetic layer 10 down to or below 1 (T·nm),and the 4πM_(s)·t is stable against the thermal treatment at 400° C.This proves that optimizing material, the thickness, and the position ofthe non-magnetic metal layer enables reduction and stabilization of theproduct 4πM_(s)·t of the free ferromagnetic layer 10.

[0225] Next, it has been investigated that oxidizing a portion of thefree ferromagnetic layer 10 achieves reduction in the product M_(s)·tusing samples described below:

Comparative Example 6

[0226] substrate/Ta(10 nm)/AlO_(x)/Ni₈₁Fe₁₉(2.2 nm)/AlO_(x)/Ta(10 nm),and

Example 10 (The Present Invention)

[0227] substrate/Ta(10 nm)/AlO_(x)/Ni₈₁Fe₁₉/NiFeO_(x)/AlO_(x)/Ta(10 nm).

[0228] The AlO_(x) films relatively close to the substrates correspondwith the tunnel dielectric layer, and the Ni₈₁Fe₁₉ layers correspondwith the free ferromagnetic layer. In addition, the AlO_(x) filmsrelatively apart from the substrates correspond with the oxide layer 14,and the Ta films relatively apart from the substrates correspond withthe top contact layer 15. The AlO_(x) films corresponding with thetunnel dielectric layer 9 is formed through oxidization of an aluminumfilm of 1.5 nm in thickness. The Ni₈₁Fe₁₉ films, the NiFeO_(x) film andthe AlO_(x) films corresponding with the oxide layer 14 are formedthrough processes described below; a Ni₈₁Fe₁₉ film of 2.2 nm inthickness is firstly deposited on the AlO_(x) film corresponding withthe tunnel dielectric layer 9. An aluminum film of 0.65 nm in thicknessis deposited on the Ni₈₁Fe₁₉ film. After the deposition of the aluminumfilm, the surface of the aluminum film is subjected to oxygen plasma.Through subjecting to oxygen plasma, the aluminum film and a portion ofthe Ni₈₁Fe₁₉ film are oxidized to form the NiFeO_(x) film and theAlO_(x) film corresponding with the oxide layer 14.

[0229] As shown in FIG. 37, Example 10 exhibits 4πM_(s)·t of the freeferromagnetic layer 10 smaller than that of Comparative Example 6. Thisimplies that partially oxidizing the free ferromagnetic layer 10achieves the reduction in the product M_(s)·t.

[0230] In order to prove that the reduction in the produce M_(s)·treduces the coercive force of the free ferromagnetic layer 10 having asize less than 1 micron, magnetizations of dot-patterned samples hasbeen evaluated. Magnetoresistance elements have been patterned intosamples of oval dots having the size of 0.5×1.0 μm, and the processedsamples have been evaluated. Patterning the structure into dots isachieved by photolithography and ion milling techniques. The sectionalstructures of the processed samples are as follows; Comparative Example7 substrate/Ta(30 nm)/Ni₈₁Fe₁₉(2nm)/Ir₂₀Mn₈₀/Co₉₀Fe₁₀/AlO_(x)/Ni₈₁Fe₁₉(3 nm)/Ta(10 nm), and

Example 11 (The Present Invention)

[0231] substrate/Ta(30 nm)/Ni₈₁Fe₁₉(2nm)/Ir₂₀Mn₈₀/Co₉₀Fe₁₀/AlO_(x)/Ni₈₁Fe₁₉(1.6 nm)/AlO_(x)/Ta(10 nm).

[0232] The AlO_(x) film relatively close to the substrate correspondswith the tunnel dielectric layer 9, and the Ni₈₁Fe₁₉ layer disposed onthe AlO_(x) film corresponding with the tunnel dielectric layer 9corresponds with the free ferromagnetic layer 10. Additionally, theAlO_(x) film relatively apart from the substrate corresponds with theoxide layer 14, and the Ta film relatively apart from the substratecorresponds with the top contact layer 15. The AlO_(x) filmcorresponding with the tunnel dielectric layer 9 is formed throughoxidization of an aluminum film of 1.5 nm in thickness with oxygenplasma, and the AlO_(x) film corresponding with the oxide layer 14 isformed through oxidization of an aluminum film of 0.65 nm in thicknesswith oxygen plasma. A Ta film of 0.3 nm in thickness is used as thenon-magnetic layer. The free ferromagnetic layer 10 of ComparativeExample 7 has a thickness of 3.0 nm, which is the minimum value achievedby the conventional technique. The product 4πM_(s)·t of ComparativeExample 7 is 2.2 (T·nm), while the product 4πM_(s)·t of Example 11 is1.5 (T·nm).

[0233]FIG. 38 illustrates magnetization curves of the free ferromagneticlayers of Comparative Example 7 and Example 11. It should be noted thatthe offset magnetic fields are cancelled to appropriately compare thecoercive forces. The free ferromagnetic layer of Example 11 exhibits acoercive force of 10 (Oe), which is smaller than that of Example 7;Example 7 exhibits a coercive force of 18 (Oe). As thus described,Example 11, which exhibits reduction in 4πM_(s)·t, also exhibitsreduction in the coercive force.

[0234] Next, it has been investigated that disposing the oxide layer 14improves rectangularity of the magnetoresistance curve of the freeferromagnetic layer, and also reduces variation in the coercive force,using samples of magnetoresistance elements having layered structures asdescribed below:

Comparative Example 8

[0235] substrate/Ta(30 nm)/NiFe(2 nm)/Ir₂₀Mn₈₀(10 nm)/Co₉₀Fe₁₀(1.5nm)/AlO_(x)/NiFe(3 nm)/Ta(30 nm)

Comparative Example 9

[0236] substrate/Ta(30 nm)/NiFe(2 nm)/Ir₂₀Mn₈₀(10 nm)/Co₉₀Fe₁₀(1.5nm)/AlO_(x)/NiFe(4 nm)/Ta(30 nm), and

Example 12 (The Present Invention)

[0237] substrate/Ta(30 nm)/NiFe(2 nm)/Ir₂₀Mn₈₀(10 nm)/Co₉₀Fe₁₀(1.5nm)/AlO_(x)/NiFe(2 nm)/AlO_(x)/Ta(30 nm)

[0238] The AlO_(x) films relatively close to the substrate correspondwith the tunnel dielectric layer 9, and the Ni₈₁Fe₁₉ layers disposed onthe AlO_(x) films corresponding with the tunnel dielectric layer 9correspond with the free ferromagnetic layer 10. All of the NiFe layersof Comparative Examples 8 and 9, and Example 12 have nickelconcentrations less than 82%; therefore, all the magnetostrictionconstants of these NiFe layers are positive. It should be noted that thenickel concentrations of the NiFe layers are different from one another,and thus the magnetostriction constants are different from one another.The magnetostriction constants of the NiFe layers of ComparativeExamples 8 and 9, and Example 12 are approximately 2×10^(−5, 9×10) ⁻⁷,and 5×10⁻⁶, respectively.

[0239] The magnetoresistance elements are fabricated through processesas described below; after forming word lines of AlCu interconnections(which correspond with the bottom interconnection 2), the word lines arecovered with SiO_(x) films, which functions as interlayer dielectrics.After the SiO_(x) films are planarized with a CMP (chemical mechanicalpolishing) technique, stacks of layers for forming the magnetoresistanceelements are disposed on the planarized SiO_(x) films. The stacks areformed through a magnetron sputtering technique. After subjecting thestacks to a thermal treatment at 275° C. with a magnetic field of 5 kOeapplied thereto, the stacks are processed through photolithography andion milling techniques into ovals having a size of 0.7×1.4 μm to obtainthe magnetoresistance elements. The aspect ratio of themagnetoresistance elements is two. The contact interfaces between thefree ferromagnetic layers and the tunnel dielectric layers within themagnetoresistance elements are positioned over the word lines. Themagnetoresistance elements are not directly connected to the word lines.The distance of the contact interfaces from the word lines in themagnetoresistance elements is approximately 200 nm. And the width of theword lines is 1 μm. The major axes of the magnetoresistance elements aredirected in the word line direction, and the crystalline magneticanisotropy of the magnetoresistance elements has its easy axis directedin the major axis direction through thermal treatment and sputteringwith a magnetic field applied thereto. The width of the word lines isapproximately sized to be as large as the width of the magnetoresistanceelements in the minor axis direction. SiO_(x) films are additionallydeposited as interlayer dielectrics to cover the magnetoresistanceelements. After forming via holes through the deposited SiO_(x) films toexpose the upper surfaces of the magnetoresistance elements, AlCu alloyfilms are then deposited to entirely cover the SiO_(x) films. Thisfollowed by forming bit lines (which correspond with the topinterconnection 3) through pattering the AlCu alloy films withphotolithography and etching techniques. The bit lines and the wordlines are perpendicular. The word lines exert strong compressive stressin the minor axis direction of the magnetoresistance elements. Since themagnetostriction constants of the free ferromagnetic layers within thefabricated magnetoresistance elements are positive, stress-inducedmagnetic anisotropy with the easy axis in the major axis direction ofthe magnetoresistance elements is induced. Therefore, the crystalline,shape-induced and stress-induced magnetic anisotropy are approximatelydirected in the same direction, and thus, the magnetoresistance elementsexhibit uniaxial magnetic anisotropy having the easy axis directed inthe major axis direction.

[0240] A set of ten samples are selected out of the magnetoresistanceelements for each of Comparative Examples 8 and 9 and Example 12, andmagnetoresistance curves of the free ferromagnetic layers of theselected samples have been measured. FIGS. 39A through 39C illustratethe measured magnetoresistance curves. As shown in FIG. 39A, the freeferromagnetic layer of Comparative Example 8 exhibits a reduced4πM_(s)·t value of 2.2 (T·nm). Although the shape-induced magneticanisotropy thereof is small, the free ferromagnetic layer of ComparativeExample 8 exhibits a coercive force of 21 (Oe), which is the largestvalue. It is considered that the increase in the coercive field resultsfrom an increase in the stress-induced magnetic anisotropy caused by theincreased magnetostriction constant thereof. Additionally, althoughexhibiting an improved rectangularity of the magnetoresistance curves,Comparative Example 8 suffers from the variation of themagnetoresistance curves. This implies that a severe interdiffusionoccurs between the Ta film and the NiFe film, and thereby leads to thevariation in the magnetostriction constants caused by the compositionvariation; these result from that the free ferromagnetic layer has areduced thickness of 3 nm, which is close to the conventional lowerlimit.

[0241] As shown in FIG. 39B, although having the smallest coercive forceof 13 (Oe), the magnetoresistance elements of Comparative Example 9experience deterioration of the rectangularity of the magnetoresistancecurves; the loops thereof are slanting. Additionally, ComparativeExample 9 suffers from Barkhausen noise. It is considered that thisreflects the fact that the coercive force is decreased due to thereduction of the uniaxiality caused by the substantially eliminatedstress-induced magnetic anisotropy, which is caused by the extremelyreduced magnetostriction constant thereof, and that the influence of thedemagnetizing field is enhanced by an increase in 4πM_(s)·t of the freeferromagnetic layer up to 3.2 (T nm) and a reduction in the aspect ratioof the elements down to 2. With respect to the magnetoresistanceelements of Comparative Example 9, reducing the demagnetizing field andenhancing the shape-induced magnetic anisotropy through increasing theaspect ratio are required to improve the rectangularity and variation ofthe magnetoresistance curves. This faces problems of an increase in theelement size and coercive force.

[0242] As shown in FIG. 39C, the magnetoresistance elements of Example12 exhibit superior characteristics; the coercive force of the freeferromagnetic layer is reduced down to 16 (Oe), the rectangularity ofthe magnetoresistance curves improved, and the variation in the coerciveforce is most reduced. It is considered that this results from that themagnetoresistance elements in accordance with the present inventionachieve reduction in the variation of the magnetostriction constant, andthereby exhibits stable stress-induced magnetic anisotropy ofappropriate strength, and also reduce the demagnetizing field andshape-induced magnetic anisotropy through minimizing 4πM_(s)·t of thefree ferromagnetic layer approximately down to 2 (T·nm).

[0243] An experiment described below has proved that themagnetoresistance element in accordance with the present invention,which have the stress-induced magnetic anisotropy controlledappropriately, achieves both a reduced aspect ratio and stabilizedproperties. Magnetoresistance elements having various aspect ratios inaccordance with Comparative Example 9 and Example 10 have beenfabricated. The lengths of the minor axes of the magnetoresistanceelements are 0.5 or 0.7 μm, and the aspect ratios thereof are in therange between 1.0 and 3.5. The samples according to Example 12, whichhave an increased magnetostriction constant, exhibit an increased ratioof the stress-induced magnetic anisotropy to the shape-induced magneticanisotropy, denoted by K2/K3; the ratio K2/K3 is increased up to orabove 0.6 for all the tested aspect ratio. It should be noted that oneshaving the aspect ratio less than 1.6 out of the samples of Example 12exhibit the stress-induced magnetic anisotropy larger than theshape-induced magnetic anisotropy, that is, exhibit the ratio K2/K3 morethan 1.0. 30 magnetoresistance elements are fabricated for eachcondition, and magnetoresistance curves of the fabricatedmagnetoresistance elements have been evaluated. This followed by thecalculation of yields. In calculating the yields, magnetoresistanceelements exhibiting abnormal magnetoresistance curves, such as thoseexhibiting Barkhausen noise and tilt loops in the magnetoresistancecurves, are defined as being defective samples.

[0244]FIG. 40 is a graph illustrating the yields of themagnetoresistance elements. The magnetoresistance elements according toComparative Example 9, which have reduced magnetostriction constants(that is, exhibit reduced stress-induced magnetic anisotropy), achievessufficiently large yield only for the samples of 3.5 in aspect ratio.This implies that the samples of Comparative Example 9 requirestabilization of the characteristics using the shape-induced magneticanisotropy. In contrast, the magnetoresistance elements according toExample 12, which enjoy sufficiently large stress-induced magneticanisotropy, achieve sufficiently large yields for the entire rangebetween 1.0 and 3.5. This indicates that enhancing the stress-inducedmagnetic anisotropy beyond the shape-induced magnetic anisotropyachieves a sufficiently large yield for the aspect ratio below 2.0.

What is claimed is:
 1. A magnetoresistance device comprising: amagnetoresistance element including: a free ferromagnetic layer havingreversible spontaneous magnetization, a fixed ferromagnetic layer havingfixed spontaneous magnetization, and a tunnel dielectric layer disposedbetween said free and fixed ferroelectric layer; a non-magneticconductor providing electrical connection between said magnetoresistanceelement to another element; and a diffusion barrier structure disposedbetween said conductor and said magnetoresistance element.
 2. Themagnetoresistance device according to claim 1, wherein said diffusionbarrier structure has a function to prevent at least one material out ofmaterials included in said conductor from being diffused into saidmagnetoresistance element.
 3. The magnetoresistance device, wherein saiddiffusion barrier structure has a function to prevent at least onematerial out of materials included in said magnetoresistance elementfrom being diffused into said magnetoresistance element.
 4. Themagnetoresistance device according to claim 1, wherein said conductorincludes at least one element selected from the group consisting of Al,Cu, Ta, Ru, Zr, Ti, Mo, and W.
 5. The magnetoresistance device accordingto claim 1, wherein said diffusion barrier structure is formed ofmaterial selected from the group consisting of oxides, nitrides, andoxynitrides.
 6. The magnetoresistance device according to claim 5,wherein said diffusion barrier structure is formed of conductivenitride.
 7. The magnetoresistance device according to claim 5, wherein athrough-thickness resistance of said diffusion barrier structure issmaller than that of said tunnel dielectric layer.
 8. Themagnetoresistance device according to claim 5, wherein said diffusionbarrier structure is a film having a thickness less than 5 nm.
 9. Themagnetoresistance device according to claim 5, wherein said diffusionbarrier structure is made of oxide of element having a free energy ofoxide formation less than those of elements included in layers connectedon top and bottom surfaces of said diffusion barrier structure.
 10. Themagnetoresistance device according to claim 5, wherein said diffusionbarrier structure is made of nitride of element having a free energy ofnitride formation less than those of elements included in layersconnected on top and bottom surfaces of said diffusion barrierstructure.
 11. The magnetoresistance device according to claim 5,wherein said diffusion barrier structure is made of oxynitride ofelement having free energies of oxide and nitride formations less thanthose of elements included in layers connected on top and bottomsurfaces of said diffusion barrier structure.
 12. The magnetoresistancedevice according to claim 5, wherein said diffusion barrier structure ismade of material selected from the group consisting of AlO_(x), MgO_(x),SiO_(x), TiO_(x), CaO_(x), LiO_(x), HfO_(x), AlN, AlNO, SiN, SiNO, TiN,TiNO, BN, TaN, HfNO, and ZrN.
 13. The magnetoresistance device accordingto claim 5, said diffusion barrier structure is an oxide layer and madeof a same material as said tunnel dielectric layer.
 14. Themagnetoresistance device according to claim 12, wherein said oxide layeris thinner than said tunnel dielectric layer.
 15. The magnetoresistancedevice according to claim 1, wherein said conductor includes: a firstconductor electrically connected to said fixed ferromagnetic layerwithout involving said tunnel dielectric layer, and a second conductorelectrically connected to said free ferromagnetic layer withoutinvolving said tunnel dielectric layer, and wherein said diffusionbarrier structure includes: a first diffusion barrier layer disposedbetween said first conductor and said fixed ferromagnetic layer, and asecond diffusion barrier layer disposed between said second conductorand said free ferromagnetic layer.
 16. The magnetoresistance deviceaccording to claim 15, wherein said first and second diffusion barrierlayers are made of material selected from the group consisting ofoxides, nitrides, and oxynitrides.
 17. The magnetoresistance deviceaccording to claim 1, wherein said diffusion barrier structure isdisposed between a layer including manganese and said conductor orbetween a layer including nickel and said conductor.
 18. Themagnetoresistance device according to claim 1, wherein said conductorincludes a first conductor electrically connected to said fixedferromagnetic layer without involving said tunnel dielectric layer,wherein said diffusion barrier structure includes a first diffusionbarrier layer connected between said first conductor and said fixedferromagnetic layer, and wherein said magnetoresistance element furtherincludes a manganese-including antiferromagnetic layer, and wherein saidantiferromagnetic layer is positioned between said fixed ferromagneticlayer and said first diffusion barrier layer.
 19. The magnetoresistancedevice according to claim 18, wherein said fixed ferromagnetic layercomprises: a ferromagnetic layer directly contacted with said tunneldielectric layer, and a composite magnetic layer disposed between saidferromagnetic layer and said antiferromagnetic layer, and wherein saidcomposite magnet layer is made of mixture including non-oxidized metalferromagnetic material as main material, and oxide material as submaterial, said oxide material being oxide of non-magnetic element morereactive to oxygen than said metal ferromagnetic material.
 20. Themagnetoresistance device according to claim 19, wherein saidferromagnetic layer and said metal ferromagnetic material included insaid composite magnetic layer is made of a metal ferromagnetic alloyincluding cobalt as main material.
 21. The magnetoresistance deviceaccording to claim 1, wherein said free ferromagnetic layer comprises: aferromagnetic layer directly contacted with said tunnel dielectriclayer, and a composite magnetic layer made of mixture includingnon-oxidized metal ferromagnetic material as main material, and oxidematerial as sub material, said oxide material being oxide ofnon-magnetic element more reactive to oxygen than said metalferromagnetic material.
 22. The magnetoresistance device according toclaim 21, wherein said ferromagnetic layer and said metal ferromagneticmaterial included in said composite magnetic layer is made of a metalferromagnetic alloy including cobalt as main material.
 23. Themagnetoresistance device according to claim 1, wherein said conductorincludes a second conductor electrically connected to said freeferromagnetic layer without involving said tunnel dielectric layer, andwherein said diffusion barrier structure includes a second barrier layerdisposed between said free ferromagnetic layer and said secondconductor.
 24. The magnetoresistance device according to claim 23,wherein said second diffusion barrier layer is directly contacted withsaid free ferromagnetic layer, and said free ferromagnetic layer has athickness less than 3 nm.
 25. The magnetoresistance device according toclaim 24, wherein a produce of a saturation magnetization and athickness of said free ferromagnetic layer is less than 3 (T·nm). 26.The magnetoresistance device according to claim 23, wherein said freeferromagnetic layer comprises a nickel-containing ferromagnetic layerincluding nickel, and said second diffusion barrier layer is directlycontacted with said nickel-containing ferromagnetic layer.
 27. Themagnetoresistance device according to claim 23, wherein said freeferromagnetic layer comprises: a ferromagnetic layer directly contactedwith said tunnel dielectric layer, and a magnetization control structureconnected to said ferromagnetic layer, said magnetization controlstructure including non-magnetic material and ferromagnetic materialincluded in said ferromagnetic layer.
 28. The magnetoresistance deviceaccording to claim 27, wherein said magnetization control structure isnon-magnetic.
 29. The magnetoresistance device according to claim 27,wherein said magnetization control structure is made of oxide or nitrideof ferromagnetic material included in said ferromagnetic layer.
 30. Themagnetoresistance device according to claim 27, wherein saidnon-magnetic material is formed of at least one element selected fromthe group consisting of Ru, Pt, Hf, Pd, Al, W, Ti, Cr, Si, Zr, Cu, Zn,Nb, V, Cr, Mg, Ta, and Mo.
 31. The magnetoresistance device according toclaim 27, wherein said non-magnetic material is segregated on grainboundary of crystals of said ferromagnetic material.
 32. Themagnetoresistance device according to claim 23, wherein said freeferromagnetic layer is formed so that axes of easy magnetization ofstress-induced and shape-induced magnetic anisotropies are directed in asame direction.
 33. The magnetoresistance device according to claim 32,wherein a contact interface between said free ferroelectric layer andsaid tunnel dielectric layer is shaped to extend in a first direction,wherein a magnetostriction constant of said free ferromagnetic layer ispositive, and wherein a compressive stress is exerted on said freeferromagnetic layer in a second direction orthogonal to said firstdirection.
 34. The magnetoresistance device according to claim 32,wherein a contact interface between said free ferroelectric layer andsaid tunnel dielectric layer is shaped to extend in a first direction,wherein a magnetostriction constant of said free ferromagnetic layer ispositive, and wherein a tensile stress is exerted on said freeferromagnetic layer in said first direction.
 35. The magnetoresistancedevice according to claim 32, wherein a contact interface between saidfree ferroelectric layer and said tunnel dielectric layer is shaped toextend in a first direction, wherein a magnetostriction constant of saidfree ferromagnetic layer is negative, and wherein a compressive stressis exerted on said free ferromagnetic layer in said first direction. 36.The magnetoresistance device according to claim 32, wherein a contactinterface between said free ferroelectric layer and said tunneldielectric layer is shaped to extend in a first direction, wherein amagnetostriction constant of said free ferromagnetic layer is negative,and wherein a tensile stress is exerted on said free ferromagnetic layerin a second direction orthogonal to said first direction.
 37. Themagnetoresistance device according to claim 32, further comprising: asubstrate; and a lower interconnection disposed to extend in a firstdirection between said substrate and said free ferromagnetic layer,wherein a magnetostriction constant of said free ferromagnetic layer ispositive, and wherein a contact interface between said freeferroelectric layer and said tunnel dielectric layer is shaped to extendin said first direction.
 38. The magnetoresistance device according toclaim 32, wherein said further comprising: a substrate; and a lowerinterconnection disposed to extend in a second direction between saidsubstrate and said free ferromagnetic layer, wherein a magnetostrictionconstant of said free ferromagnetic layer is negative, and wherein acontact interface between said free ferroelectric layer and said tunneldielectric layer is shaped to extend in a first direction orthogonal tosaid second direction.
 39. The magnetoresistance device according toclaim 22, wherein stress-induced magnetic anisotropy of said freeferromagnetic layer is stronger than shape-induced magnetic anisotropyof said free ferromagnetic layer.
 40. The magnetoresistance deviceaccording to claim 39, wherein said free ferromagnetic layer has a majoraxis and a minor axis perpendicular to said major axis, and an aspectratio, defined as being a ratio of said major axis to said minor axis,is equal to or more than 1.0, and is equal to or less than 2.0.
 41. Themagnetoresistance device according to claim 23, wherein said freeferromagnetic layer comprises: a first ferromagnetic layer directlycontacted with said tunnel dielectric layer, a composite magnetic layerconnected to said first ferromagnetic layer, and made of mixtureincluding non-oxidized metal ferromagnetic material as main material,and oxide material as sub material, said oxide material being oxide ofnon-magnetic element more reactive to oxygen than said metalferromagnetic material, a second ferromagnetic layer including nickel,and connected to said composite magnetic layer, said secondferromagnetic layer being magnetically softer than said compositemagnetic layer and said first ferromagnetic layer.
 42. Themagnetoresistance device according to claim 40, wherein said firstferromagnetic layer and metal ferromagnetic material included in saidcomposite magnetic layer are made of metal ferromagnetic alloy mainlycontaining cobalt.
 43. The magnetoresistance device according to claim1, wherein said free ferromagnetic layer comprises: a firstferromagnetic layer directly contacted with said tunnel dielectriclayer, a first composite magnetic layer made of mixture includingnon-oxidized metal ferromagnetic material as main material and oxidematerial as sub material, said oxide material being oxide ofnon-magnetic element more reactive to oxygen than said metalferromagnetic material, a second composite magnetic layer made ofmixture including non-oxidized metal ferromagnetic material as mainmaterial and oxide material as sub material, said oxide material beingoxide of non-magnetic element more reactive to oxygen than said metalferromagnetic material, a non-magnetic layer disposed between said firstand second composite magnetic layers to achieve antiferromagneticcoupling between said first and second composite magnetic layers. 44.The magnetoresistance device according to claim 43, wherein said firstferromagnetic layer, said metal ferromagnetic material included in saidfirst composite magnetic layer and metal ferromagnetic material includedin said second composite magnetic layer are made of metal ferromagneticalloy mainly containing cobalt.
 45. The magnetoresistance deviceaccording to claim 1, wherein said conductor includes a second conductorelectrically connected to said free ferromagnetic layer withoutinvolving said tunnel dielectric, wherein said magnetoresistance elementfurther includes a magnetic biasing element providing a bias magneticfield for said free ferromagnetic layer, wherein said magnetic biaselement includes comprises: a magnetic bias ferromagnetic layer, and amagnetic bias antiferromagnetic layer including manganese and connectedto said magnetic biasing ferromagnetic layer, and wherein said oxidelayer includes: a first oxide layer disposed between said magneticbiasing element and said free ferromagnetic layer, and a second oxidelayer disposed between said magnetic biasing element and said secondconductor.
 46. A magnetoresistance device fabrication method comprising:a step of forming a fixed ferromagnetic layer, a step of forming atunnel dielectric layer connected to said fixed ferromagnetic layer astep of forming a first ferromagnetic layer on a contact surface on anopposite side of said fixed ferromagnetic layer, a step of modifying anopposite portion of said first ferromagnetic layer, said portion beingpositioned on an opposite side of said contact surface.
 47. Themagnetoresistance device fabrication method according to claim 46,wherein said opposite portion is modified to be non-magnetic.
 48. Themagnetoresistance device fabrication method according to claim 46,wherein said step of modifying includes: a step of nitrizing oroxidizing said opposite portion.
 49. The magnetoresistance devicefabrication method according to claim 46, wherein said step of modifyingincludes: a step of forming a non-magnetic metal layer made ofnon-magnetic metal on an opposite surface out of surfaces of said firstferromagnetic layer, said opposite surface is positioned on an oppositeside of said contact surface, and a step of achieving inter-diffusionbetween said first ferroelectric layer and said non-magnetic metallayer.
 50. The magnetoresistance device fabrication method according toclaim 49, wherein said material includes nickel.